can anyone point me in the direction of a coversion table for vehicles from 40k to RT? or does something of the sort not even exist?

Doesn't exist. Liber apocrypha: vehicles had a bunch of military vehicles, as does deatheatch rites of battle, into the storm and battlefleet koronus. What stats were you after in particular?

Well, my players seem to be ammassing an army and knowing them they're gonna want several armoured regiments, and support units on top of an large number of mechinised infantry battalions. So what i would be looking for would be the assorted Leman Russ variants, cargo 6's and 8's hell maybe even cargo 12's Chimera, salamander scout vehicles. I just grabbed the Tanks of the imperium from Dark Reign so really what i'm intreseted in at this point is the cargo x's and chimera/hellhounds.

thanks.

Chimera Armoured Personnel Carrier
The Chimera is a supremely durable tracked troop transport, with the ability to carry up to twelve passengers along with a crew of three. It is durable with a heavily armoured prow, and well-armed with a turret-mounted multi-laser and a hull mounted heavy bolter as standard. Six lasguns are attached to the sides to be fired by transported troopers remotely (so they do not need to expose themselves when firing). A top hatch can be opened to afford more weapons fire, with the main rear hatch being used for egress and entry. The hull is air and watertight, making the Chimera both amphibious and survivable in a toxic environment.
Type: Ground Vehicle
Size: Enormous
Armour: Front 27, Hull 18, Rear 16
Traits: Amphibious
Narrative Speed: 35 kmph/55 kmph
Combat Speed: 5/20/40/60/80
Handling Modifier (Drive (Ground Vehicles)): 0
Armaments: Forward hull heavy weapon mounting, turret heavy weapon mounting, six (three each side) side hull basic weapon mountings.
Crew: 1 (Driver), 2 (Gunner)
Passengers: 12
Access Points: Top and rear hatches

What you are looking for sounds more like a working mass combat system rather than a stats for individual vehicles though.

Which makes me wonder if the horde rules could be used for vehicles effectively? Id say no.

But that has more to do with the fact that any type of larger engagement which involves larger than company sized forces should be combined arms and produces stats to be used that give you stats for mixed troops, unless you want to break out the 40k tabletop and declare a single Leman Russ to represent a platoon of them etc.