It's about 15 provinces that you could colonize freely once you block off the Siberian coast. They matter later in the game when your colonists run out of places to colonize around 1700s (depending on how aggressive you colonize).
15 provinces aren't going to matter. In my Ayutthaya campaign, I held SEA, China, Japan, Korea, some of Indonesia (the parts you can see from the start, I don't have explorers), and maybe 1/3 of India, and that was enough to get me the 400 provinces achievement. I seriously doubt that you'll even need to touch Siberia to get 500.
You can't vassal forever and there will be a point where you have to annex. That's when you don't want countries like Russia to be in a coalition against you. You can annex many provinces from Russia in one go, but that'll mean they will be in a coalition against you for a long long time. Of course, you can do DDRjake's coalition burner but why go through all the trouble if you can just plan it out early in the 1500s and seize that one colony and solve the Russian trouble once and for all.
Yeah, but that's exactly why you want Russia to eat up all the Hordes. You let them take over all that land, and then you vassalize them and force Russia to release them all. Since Russia is so big, you can probably do it all in one go, too.
Also, Russia being in a coalition against you isn't necessarily a problem. I've found the best strategy to expand quickly is to pick one area as your main focus, preferably somewhere that's culturally, religiously, and/or geographically isolated, like India, and just hammer down on that area, going to war and annexing as much land as you can every 5 years as truces end. Everyone in that region will form a coalition against you, but once they do, it probably won't spread any further due to coalition mechanics. Just keep working on them until they're all gone, because they can't be in a coalition against you if they don't exist. Then, pick another area, and start working on that as well.
Aside from your main focus, of course you'll also be expanding in other areas, but you just need to make sure that you don't provoke coalitions in those areas. For example, you might pick India (the Hindu parts) as your main focus, and grind them down, while also expanding in China and Japan through core returns. In the same way, you could have Russia be in a coalition against you, and simply let them be in it on their own while you grind them to dust in war after war, and as long as nobody else joins the coalition (and why would they? Nobody cares about Russia), it's not a problem. You might start annoying the other European powers once you start forcing your way into Russia proper, but that just means you fight Russia last.
Anyways, here's my Najd campaign so far:
Only 1536, but I think I'm on track to getting the achievement. I've got 79 provinces so far, plus 19 extra held by vassals, so that's 92 provinces of growth from the 6 I started with, for an average growth rate of exactly 1 province per year. I'll need to pick the pace up a bit to make it in time, but now that I'm comfortably powerful I should be able to really start growing fast, especially once I hit China.