If you start Magic Online today, is there a way to acquire Vanguard cards that do not come with the client but have been released in the past? Can they be bought or traded?
Yes, they can be traded.
I don't believe they are sold by Wizards, when I used to play they were given as tournament or set release perks.
Ah, looks like nowadays new players get a booster pack of default Vanguard cards.