Why would he sign on to be number 2?
MLS contracts are also with MLS rather than the clubs, making them more secure than a lower league European one. They're also either semi or fully-guaranteed (first-year players on semi-guaranteed contracts can be released before a certain date, after your first year a contract becomes fully-guaranteed). The least Stefan could be paid is USD$65,000, (well, $32,500 because it's a half-season) rising to $67,500 next year. It isn't a lot in the grand scheme of things, but I wouldn't be surprised if it compares favourably to 3.Liga salary. For comparison, Gleeson is on USD$110,000.
With regards to his future playing time, the contract is until the end of next year and with team options on 2019 and 2020, which makes it look like Vancouver see him as a potential long-term option and a replacement for Ousted.
Another key thing to note is that he has been signed by the MLS team rather than the affiliate, so he's only going to play USL if the 'Caps sign another reserve keeper, he drops to no. 3 and gets loaned to Whitecaps II.