Well, in singleplayer game bots often use vehicles. But in multiplayer bot in a tank or whatever is a rar sight... Is there any way to make bots interested in vehicles during online game?

I'm pretty sure the reason is because players tend to immediately take the vehicles so they can troll.
I remember one time, I got in an AT-AT on Hoth MP and spawn killed everyone!

Well, it is not because of that. I play LAN and there are only 2 human players. Even when we don't get to the tanks or whatever, bots goes everywhere walking. In singleplayer they take all tanks.

Usually bots get in the vehicles on single and multiplayer when i'm playing. Same with mods and the stock game. It could be the difficulty level or something, my setting is on elite.

I alsways play on elite too. But in mp bots do it less often...

It may be some hardcoded thing so that on mp the bots don't take all the tanks and leave none for the actual players.