The tank, weighing in at 130 points base, is a Heavy support option for all Space Marine factions (interestingly, this includes Grey Knights) and Chaos Space Marines in Warhammer 40,000, and for the Space Marine Legions in 30k. Since I have the Chaos codex to hand, I will focus it's use in a Chaos army, though the advice here can be applied ot all.
The Rapier Vindicator differs from the standard siege tank, in that the Demolisher Cannon has been swapped out for a Laser Destroyer Array. You loose the cannon's blast template and one strength point, but you benefit from doubled range, better AP and higher accuracy via Twin Linked. This mades the tank perfect for taking out tanks, despite the strength loss.
Some people might compare this to a Lascannon armed Predator. They might point out that a Predator, with a twin-linked Lascannon and 360 degree firing arc, is about 30 points less, so why would you run this? Well, the answer is overcharging. This relic tank can supercharge its main gun via Capacitor Fire to fire twice, if it doesn't move. It also has the option to Overcharge Fire to get three shots, but has a small chance of taking damage. This makes it an incredibly potent anti-tank weapon, with up to three twin linked shots with ordinance. The heaviest of battle tanks and monsters, even super-heavies, should fear this tank, and this ability makes it a worthy choice over the Predator.
Using this tank against units is practically pointless, so dedicate this vehicle against any armour, fortification or large creature in sight. If you absolutely have to fire at infantry, aim for the tougher units like Terminators and other models with higher armour values. Use Overcharged Fire to make the most of the gun.
Make use of cover with this tank; whilst its front armour is good, it suffers from having the Rhino's weak side and rear armour. Use cover to avoid exposing your weak spots, and stay far back on the table to avoid early charges. Also, it might we worth having a basic squad posted nearby to repel infantry.
In terms of a loadout, it's pretty good stock, and there aren't really any other weapons to bolster its anti-tank capability. Personally I'd attach either a twin-linked bolter, Storm Bolter or Havoc Launcher for a slightly boosted anti-troops performance, and to help prevent the main gun getting destroyed.
Apart from that it doesn't really need much upgrading. A siege blade is a good idea if you ever plan to move the tank, but its long range should have you covered. For Chaos players, daemonic possession might be very worth it on this tank. Sure you go down to BS3, but you have twin linked to help you out. Having said that, this tank has less fear of being stunned than a normal Vindicator, since it can still fire and benefit from twin-linked...when overcharged, and statistically speaking, at least one of your six dice will hit.
With this advice on using the Rapier Vindicator, hopefully you'll consider this awesome blast from the past for your forces
(Images from Forge World, used for review purposes only)