Welcome to Librarium Online!
I was bored, so I threw together a small Tyranid list, coming in at exactly 500 points.
Tyranid Prime w/ Deathspitter, Lashwhip and Bonesword
Tyranid Warrior Brood w/ three Warriors, Deathspitters
Tervigon w/ Catalist
Supported by the Venomthrope's cover save and the Tervigons psychic powers, the bugs move up the bored, softening the enemy up with ranged fire until they can assault, nothing special really.
Right now my biggest concern is the lack of anti armor, would it be a good Idea to drop Catalist to give a Warrior a Venom Cannon?
I think this is about as good as you're going to get for 500pts short of running a tyrant (which is of quesitonable nessecity at this point). Venom cannons are poor for killing vehicles, their strength just isn't high enough for what you need. I would drop the Venomthrope for a Hive Guard if you're worried about vehicles. Positioning will take a large roll in your vehicle killing, against side/rear armor of most vehicles you face at 500pts, the deathspitters should be able to floor them.