I like having all the vechs in OP2. If I had one vech that did everything, I think that would be less fun.
Having a multi-role vech isn't a bad idea. But I don't think having a multi-role vech that can haul ore, be a heavy weapons platform, or a repair vech works for me. Maybe have it able to fully field repair vechs and able to help a convec during construction. Or something that could sample for ore and haul it (like a surveyor / cargo truck).
Or maybe more appropriate, a scout/surveyor combo. Because neither one are very useful alone after they've served their initial purpose.
But again, with all that being said, I like having several kinds of vechs rather than an all inclusive. If we want a changable vech, perhaps make it possible to change the turret type of an existing vech at the garage (for a price). So if you have a bunch of laser/mic sitting around, you could upgrade some to RPG or EMP if you want. (This would probably be more useful in a campaign type of game where units carry over.)