That makes no sense, why would this be?
Standard FIFA tie-breaking rules. You tally up the total goals scored by each side in two legs (one home, one away). If it's tied, but one team scored on the road (which is "hard to do" in soccer terms) more than the other, that team wins. It's how the Whitecaps lost the 2013 Voyageurs' Cup to Montreal, and why they were in a great position to win it this year, after drawing 2-2 in Montreal (a game I attended).
Isn't home field kind of a disadvantage since you have to outscore your opponent or you lose due to away goals?
Well there is one game in each city, so both teams therefore have the same opportunity to win on away goals... there is no real "home field advantage" to either side overall (as compared to a best-of-odd-number-of-games series), except that it's a bit of a psychological edge to be at home for the second game when you know exactly what you have to do. I mean, Vancouver could have taken advantage of this rule, if they had so much as notched a 1-1 draw in Portland (or even lost 2-1, say). In the former case, they could play for the 0-0 tie at home and win.
The reason why this sort of prejudices home teams is because, historically, soccer teams just have a hell of a time scoring on the road and routinely play for the draw. In a winner-take-all game you want to be at home, so the assumption is that a home team victory is still the most likely outcome. And to be fair, all else being equal, the Whitecaps are probably in a slightly better position than Portland by getting a saw-off there -- essentially they have reduced it to a one-game series in Vancouver only, where the historical odds say they are in the more comfortable position and more likely to win. But by not scoring they have exposed themselves in a draw situation other than 0-0.
Note that if the game in Vancouver finishes 0-0 and they go to extra time, the away goals rule still applies, so a 1-1 draw in extra time loses it. (The tradeoff being that the Whitecaps are still at home for the extra 30 minutes).