SSGSS (Super Saiyan God Super Saiyan) Goku and SSGSS Vegeta are billed as early unlock bonuses for pre-ordering Dragon Ball Fighter Z on both the PlayStation Store, Steam, and XBox Store, but here's some good news for you who don't want to pre-order. Both blue Goku and blue Vegeta are totally unlockable by just playing the game and you can do it pretty early on, even without becoming a super master at the game.
There are two ways to unlock them.
One is via the in-game currency, Zenny, which is fairly easy to get. The game seems to be very generous with the Zenny. If you play arcade, it gives you over 10,000++ Zenny. Going into training gives you 2,000. The trials gives you another 3,500. The story mode also gives you a ton of Zenny, which can go from 200,000 to 300,000 depending on the path you take in each map. (Pro-tip: do as many fights as you can in story mode, it's fun and it'll level you up faster). In other words, practically everything you do in Dragon Ball FighterZ gives you Zenny.
To unlock SSGSS (or SSJB, which stands for Super Saiya Jin Blue) Vegeta, you'll need to earn 300,000 Zenny. For SSJB Goku, you'll need to earn 500,000 Zenny. And yes, it's cumulative and the game tracks it so you can spend your Zenny at the store for cosmetic and customization items as you please.
The other way, my preferred way, is by beating the two Arcade mode courses, Extreme Gravity Spaceship and Hyperbolic Time Chamber, on hard mode and getting a rank of A or higher.
The difficulty scales the better you do in the previous match. If you score an S or higher, you usually move up the pyramid-like structure of Arcade mode. Getting a rank of B or C moves you laterally, and anything lower than that moves you down, which gives you weaker opponents but of course your overall grade tanks. The enemy AI is pretty easy especially if you spam Super Dash a lot, but the higher levels make the enemies deal way more damage while taking much longer to take down.
That's pretty much all there is to it.
Of course, it'd be much quicker to just pre-order, and you do get a ton of other goodies to go along with it like exclusive SSGSS Goku and Vegeta avatars with the glowing blue aura and some emotes, but at least this way, if you can't buy the game right now, you'll have a chance to catch up.
Now, the next question is - are SSGSS Vegeta and Goku better than Super Saiyan Vegeta and Goku?