Apparenatly, there are some maps where you can actually insert vehicles in the infantry path
Lakeside
Step 1 : Find the cavern at the center of the field filled with Tiberium Crystal. Bypass the trees carefully
Step 2 : Pass the cavern through the river carefully. Should not take too much effort
Step 3 : You're there, from then on, you can either follow the trail through the trees, or push your way through the waterfall. You can do that, almost. My arty got stuck. Do this if you're as stubborn as the RenX harvester
Step 4 : As you arrive at enemy's backdoor, you can shell their structures, but you cannot go inside. The only way to get out is trace your way back out
Applicable to every vehicles EXCEPT Mammoth, for its' obviously wide size
X-Mountain
Aaah, this new map with all its' collection of glitches. Apparently there's another that might or might not break the game. Untested to all tanks, but might be applicable to every tanks
Step 1 : Search for the spot nearby the barricade by the bridge from Nod's side (GDI's side is still affected by vehicle blocker)
GDI's side is blocked
Nod's side however, is not
Step 2 : Cross the bridge and then find some collision-glitched rocks if you drive something as sizeable as a mammy. Should not be hard
Step 3 : Have fun shelling enemy's structure!
Humvee can apparently enter the bunker on the trail, as shown here
As bonus, I've stacked 5 Mammoths on the bridge. The last one was stuck on the stairs, so I had to change the gravity to 1 to lift it up to the air briefly (be careful!)
Do not use for exploit. I'm just doing this for the fun of it