Vindicators carry the most powerful weapons in the Space marine arsenal, the Demolisher Cannon; a str10 large blast that goes through even Terminator armour. For something to be this powerful, there has to be drawbacks.
Mainly, it’s the Vindicator’s range. 24” means that it will endure at least one turn of no shooting to get within range to do the damage. That’s one turn where every gun is pointed towards the Vindicator as nobody wants that kind of damage output landing on them. That’s why it’s best to take two. These guys are very powerful so are high priority targets. If there’s one, it’s nearly always the prime target. If there are two, the fire has to be split between the two or concentrated on just one and increases their survivability. Three increases this chance but the third one can be replaced with some other high priority target, such as a Land Raider.
Another major weakness, and probably the more popular reason to dislike them, is the fact that it’s a blast. That means every shot scatters 2D6 minus the BS of the tank. Now scatter is pretty bad but think of it this way. If you roll 2 6’s for the scatter you’re going 8” in the direction the scatter dice goes. 8” isn’t all that big to be honest (insert sexual innuendo here) and it is very easy to have your assaulting units doing their job and the Vindicators doing theirs with no risk to your own men. Unless the enemy can and does deploy everything within 10” of each other you can assault units easily and fire the Vindicators at anything roughly 6” away from the assault happening.
A majority of lists have a static firebase (auto-las predators, devastators/ long fangs, lazerbacks etc) along with an assault element. You hit the assaulting element with Vindicators before your units get into contact with them, to soften them up, then when they are held up in an assault, you can target the further away “firebase” units. The scatter can be easily avoided and therefore one weakness taken out of the equation.
The vindicator has all the same options as all the other vehicles in the codices apart from one exception; the Siege Shield. This upgrade allows you to completely ignore the effects of driving through terrain which allows the vindicator to bring its cannon to bear much more easily. This is a very nice upgrade but if points are a problem then Dozer Blades for half the price are almost as good. They allow you to reroll the dangerous terrain roll which, although not as good as ignoring it completely, is nice for the cheaper cost.
Extra Armour I would recommend on vindicators from every codex apart from the Blood Angels codex. Because of the short range, it is vital to get the vehicle up there quickly and being able to move after taking damage is helpful. The Blood Angels Vindicators are fast so one turn of not moving isn’t as bitter as it is for other codices.
The extra Storm Bolter is a bit unnecessary, personally, as you already have a very powerful blast coming down on the enemy. Two extra str4 shots isn’t going to do much. Points could be much better spent on Dozer Blades instead.
One of the best ways to use Vindicators is to draw fire away from the softer parts of your list. One on each flank is something that the enemy has to worry about and that will take his fire away from things like Rhino squads moving up to take an objective etc. Another solid tactic is, if using a rock unit such as a Land Raider with terminators, is to have one vindicator on each side of the land raider and cover the sides from fast moving melta units, counter assault units when the terminators are in an assault, basically anything that can cause trouble for a rock unit like that.