I don’t think this has been mentioned yet, but I’ve noticed that for me at least, Obsidian reopens whichever window was the last to close when it was last quit, and the vault selector counts as one of those windows.
So, for me at least, if the last window to close was a vault, that is the vault that will open when I launch Obsidian again. But if I instead open the vault selector first, close my vault, then close the vault selector, then the vault selector is what will open when I relaunch Obsidian.
What I personally do is keep the vault selector open in the background before I start working. When it comes time to quit, I close my vault window first, revealing the vault selector in the background, which I close second.
I don’t think this workaround has been mentioned yet, so I hope this will work for others too!