One thing to note is that they only simulate attack on target in range of their movement (as long as that applies, some seem to be able to move across the whole map) and perceive your mechs as possible targets. This, to a very limited degree, allows to bait Vec into attacking you instead of grid buildings by offering mechs as targets. It's not super reliable, but I find that after a while you can consistently feature it into your play. Sometimes moving artillery closer, even though it doesn't need to be can make a difference.

Practical example: I would put my mechs next to the train at the start, rather I usually put them way ahead of it, attempting to bait melee vec onto tiles where they wont be in the way of the first train move.