Is it possible to download PSN games (PSVita, Vita-compatible PSP games and minis) via PC software (i.e. Media Go) and then connect Vita with the PC and transfer Vita-compatible games (either by using Media Go, Vita Content Manager Assistant or some other way)?
No, it's not possible to do so because PS-Vita games can only purchasable via a PS3 with a valid account. You can only transfer PSN games via a PS3 (this restriction is not applicable for media contents, videos, music etc.)
See here http://mediago.sony.com/enu/features/devices for all the devices that supports MediaGo to transfer content, and PS Vita is not listed.