I would go with lightning. Smaller ,more maneuverable, and the viper isn't that bad of a gun for a default. The HE cannon is ok but it really doesn't hold too much ammo. I always seem to run out towards the end of a base capture or right when something big is about to happen.
Rule of Thumb: Unless you're a Prowler, the AI job is done better by anything but your MBT. PPA harasser > PPA Magrider. AI harasser > AI Vangaurd and Viper/HE Lightning > EVERYTHING.
a lightning has a higher rate of fire as it can reload quicker, but a vanguard has higher direct damage and a larger splash radius for its HE/HEAT cannons. basically, choose what you prefer
Lightnings are a solid choice, but really a good Vanguard loadout will be overall more useful. If you pull a Vanguard with HEAT/Kobalt or HEAT/Basilisk, you're going to kill quite a few infantry while being considerably more durable than a Viper Lightning (and better equipped to take out enemy hard targets when you reach them.) I haven't tried C85 Modified since the patch, but maybe that's actually useful now too.