Considering I play VIP/Lovers a lot, I have a few opinions on this.
Coven 'never' wins because they don't communicate effectively and they don't use their roles in combination with one another and (something that gets me all rattled at times) they don't use their own roles in a way that maximizes its effectiveness.
In my experience in playing I have made a list of the most effective tactics for me to play in VIP but of course, it's situational.
I will just say the best uses that I've found for the two Coven roles that are in every game.
Medusa is very bad late game but incredibly powerful early game where nobody knows each other. If the Medusa does a really strong bait D1 and CL chooses to control a random into them you could get a lot of claim space which Coven DOES NOT have in the VIP rolelist (if you do that, to minimise risk, have the PM heal the CL). They are the last to have the Necronomicon so after the first few nights, they are useless so throw them under the bus and push them to either throw suspicion off another important role like Poisoner or Potion Master or just to confirm yourself as town. Pretty much any Coven role that can kill a specific target is more important than this role in this gamemode.
Coven Leader is by far the most powerful and most important Coven role in this gamemode. Crusader almost always claims 'On VIP' D1 and you can use that to your advantage. You can control them off of VIP and use another Coven role to sweep town for the VIP.
"But what about if another Crusader is on them?!", did you know Crusader gives Powerful protection? Control them into yourself and you'll be good (make sure other Coven are not on you such as PM healing because they will probably die). Keeping the TP off of the VIP is a smart move, but depending on the situation, another tactic may be useful.
Potion Master is important because they can heal a Coven if they're going for a risky attack and they can attack a practically confirmed VIP while the Coven Leader controls the Crusader or TP away. The Investigation ability isn't very important in this game-mode.
Though outside of good ways of playing your specific roles, there are some things that every Coven role can do and that is town reading.
Brilliand wrote:VIP mode kinda reverses the standard formula: it's the Coven that needs to "scumread" the Town. Guess who the VIP is and you win, almost instantly.
I doubt that the Town has gotten so good at hiding the VIP as to be impossible to read.
Brilliand is correct in saying that it reverses the standard formula and you can use the fact that EVERY town knows who the VIP is to your advantage. Unless they gamethrow, you can bet that the Tracker and Sheriff will never visit the VIP. Neither will an Escort, or a TP (excluding Crus) if the Crusader is alive. If a Psychic uses 'VIP' in their results they are likely not VIP and thus you can use their results to narrow down who isn't the VIP. If somebody is not grilled when having their name on an evil Psychic vision, they could be VIP. Use that to narrow down your list of who is the VIP and once you've found them, control away the Crusader and whack them with an attack.
Also if there's one person nobody's ever pushing, take suspicion onto them as being the VIP and act accordingly. Also if you don't control the Crusader away and the either the Pirate or a fellow coven dies to Crusader (if the Pirate keeps a will), you pretty much know who's the VIP so control away.
This is just from my experience and when I am playing with these in mind and when Coven do what I suggest I tend to win quite often. If you disagree or want to add something to this please lemme know.