Viking has a special place in the Terran match up, and its role has evolved in Legacy of the Void. I will discuss how Viking has become a part of the core composition.
The match up can generally be categorised into early, mid, and late game. The mid game is where both players focus on producing the units that make up their main composition, for example, a bio player will produce Marine, Siege Tank, and Medivac. Viking played an important role through out the whole game for mech, but it usually did not have a place in the main Marine Tank composition in Heart of the Swarm and early Legacy of the Void. However, although Viking was not produced in the mid game, it was often produced in the early and late game for different purposes.
In the early game in Heart of the Swarm, Viking was the main source of anti-air, which was crucial in defending against two main sources of attack: Banshee, and Marine drop with Hellion variations (briefly discussed the main variation here). A single Viking with a Missile Turret in the mineral line is sufficient to defend against Cloaked Banshee. A Viking was also critical in defending against any 1-1-1 drop variation, because it put a timer on the Medivac and this lowered the effectiveness of the attack drastically. Essentially, it was the most common unit produced from the Starport for the player who had the defender role. Generally, players stopped further Viking production after three to four Vikings, as the subsequent Vikings added little value and they were also an opportunity cost for Medivac production. These Vikings did not get replenished when they got destroyed.
Occasionally, some TvT games did not end with the mid game composition, and players had to transition to a more late game composition that focus on air units. Vikings would then be produced again from multiple Starports.
In sum, Viking had an important niche role, but it was never part of the core composition. See the image below.
In the current metagame, Raven is the most popular air unit choice in the early game. The Auto Turret is strong against those Cyclone with Medivac timing attacks, and it also has good harassment potential. The fact that Raven is effective as a harassment option, it shows how unpopular Viking is. If you know that opponent has a Viking to anticipate attacking air units, you will not send in your Raven. Just to be clear, I’m not say Viking is not used in the early game, but it is simply not the priority now. Thus, you often see players produce one or two Raven before a Viking was queued in the production.
Cyclone is another reason for this change. As I have discussed before, Cyclone plays a pivotal role in the early game for TvT, and it also has a decent anti air ability that works well with Raven. Thus, Cyclone sort of fits the role of both Hellion and Viking in the early game. Of course, Viking is still a better option for what it does, but the circumstances allow players to prioritise Raven first.
In the vod above, you can see that aLive went for the defender role with a Raven, while GuMiho went for the attacker role with Banshee. aLive only made a Viking after the Raven to shut down potential Banshee, after he knew that GuMiho also went for a one-base build (Banshee is a common option for one-base). Then, aLive went back to make to another Raven, which is considered really strong in the early game right now. Interestingly, GuMiho made two Ravens after the Banshee, and this again shows the priority to produce Raven in the early game.
In the vod below, INnoVation made a Raven first as usual, and you can see how that could be effective against Cyclone and Medivac timing. The Raven plus Cyclone combination works just fine against drop. Watch the part when INnoVation drops an Auto Turret when aLive tried to drop his units out of the Medivac (timestamped already).
I actually had a hard time finding a TvT game whereby Viking is the first unit made from the Starport for a typical opening, and Jjakji’s game below is the only one I can find.
Broadly, there are two TvT styles in the mid game now. One is the conventional Marine, Tank, and Medivac composition that tries to out manoeuvre the opponent and siege up in important locations. This is the basic style of TvT since Wings of Liberty, and it was still the dominant style in the earlier Legacy of the Void days. The second style sacrifices mobility for more control by mixing in a certain number of Vikings in the main composition. The main composition still remains the same with Marine, Tank, and Medivac, but the number of Medivacs usually is capped at around six. It is difficult to split up some units for drop and have enough Medivacs with the main army when there are only six Medivacs. Thus, the decision to limit the number of Medivac to six decreases the mobility of army from a tactical perspective. The second style has become more popular that it is arguably the main style, and I will focus on this relatively new style.
As you can see from the image above, Viking production starts in the later part of the early game, and it stretches across the hold mid game. The Viking production is later than the old style, because the Starport is producing Raven(s) first. The default approach is to make four Vikings first, then switch to Medivac production. Subsequently, you switch back to Viking production after you have four to six Medivacs, and replenish the Medivacs as you lose them. I put a question mark at the late game section, because there is no true late game transition for TvT in Legacy of the Void. The vod below shows the contrast of the two styles with TY used the new style, while Ryung used the old one. After TY had made two Raven, he switched the Starport onto the Reactor for Vikings, and switched the Barracks onto the Tech Lab for Stim. This is a common sequence when the player wants to move to the mid game, and it is also something I had previously discussed.
There are two main reasons for this style to be developed after the patch 3.8 redesign. The first reason is the removal of Tankivac. Tankivac was just too important in the match up, and the early to mid game transition was strongly influenced by it. There was little incentive in making Viking apart from the purpose of early game defence. Then, logically, why was this style not this prominent before Legacy of the Void when there was no Tankivac? This leads to the second reason, which is Liberator’s ability to capitalise on air advantage. You should make a pair of Liberators once you have the air advantage, and you can use that to make a push. Therefore, as I have mentioned earlier, this style trades mobility for control. See the vod below.
The early to mid game transition Viking production is not new, and it was usually used for an early Tank push while opponent is transitioning to mid game. The vision and air advantage from the unusually high Viking count made it extremely hard for the Terran opponent to engage the Siege Tank at that stage of the game. Nevertheless, the attacking player still halted Viking production, and went back to the usual Medivac production. This is because it is difficult to turn air advantage to actual advantage. At best, you force opponent to unsiege the a few Siege Tanks, while you leap frog forward a little. This takes way too long, and this gives opponent the opportunity to just drop at your base.
Another merit of this style is its ability to restrict doom drop. Just think about it, do you dare to doom drop when you know opponent has six Vikings and two Ravens waiting in air space? However, the conventional Marine, Tank, and Medivac still has a place in the metagame, especially in the bigger maps. As you can see from the vod above between TY and Ryung, Ryung used the conventional style for big map (game 1), but switched to the contemporary style for mid size map (game 2).