Although, if there is a need for a vehicle that can traverse over Lava, TH300's suggestion about degrading HP is a great idea. Also, if a vehicle is moving over a lava flow, it's movement will very likely be very slow. Lava isn't exactly known for it's traction.
What about some form of a areo boat type thing (I forgot the name... they have like the hole in the middle below the deck, and pontoon thingies on the sides connected by a piece below the hole. It's some really fancy boat that goes like 80MPH+... ( I think).
(different but related idea below)
Anyway, a variation of this could be applied to lava. It is like that boat they use in Antarctica to break ice. It would do the same to crush the rock, But it would melt the rock using some form of a weapon on the front edges, turining it into a 'slushy' like material. The boat would just use some form of a weapon, explosions, or a jet to propel itself from behind.
Oh, and this could also be used for normal ground:
1. It could be a new weapon, causing dirt to melt and move in the opposite direction it's being fired at, so
Shoots here hits sand
X ------------***->->
beam goes then sends dirt in the same direction after melting it.
2. A new version of a robo-dozer
It would melt it after X number of marks or whatever, then would cause a 'mini' lava flow to go and surround what it is attacking. If it is a Structure, it will just make the structure stronger because by the time it reaches it it woudl harden around it and hold it into the ground. If it's a vehicle, it would be like permanent stickyfoam. You could use another one to free the vehicle, and have little 'burrowers' that dig things up if they are buried.
I like the idea of an improved robo-dozer, But It doesn't really need to be able to dig through lava -_-. Just let it clear land, and it would create a big pile of dirt.
Dirt piles created by dozers could have ore or other materials in them, so they could be mined, or turned into walls to build a 'fortress'. With enough of it, you could 'technically' make a mini mountain
.
Tornados would pick loose dirt up and would sling it everywhere, damaging everything slightly within a ___ <units> radius.
Note: the dirt isn't sandy, it's like mud, but harder.