Vlad hit .290/.317/.416 last season for Baltimore, hardly inspiring numbers for a designated hitter, especially when you factor in the 23 double plays he grounded into.
The most realistic expectation is that Guerrero could provide a platoon bat at DH and a pinch-hitter off the bench. While his splits were pretty even in 2011, he did hit .338/.395/.536 against left-handers in 2010 when he was with Texas.
So far, however, the Jays have actually hit better against lefties than righties:
Blue Jays versus RHP: .232/.305/.387
Blue Jays versus LHP: .248/.330/.432
That doesn't mean there isn't room for improvement, most obviously by benching Adam Lind permanently against lefties. Lind is a career .217/.263/.345 hitter against left-handers. Basically, he should never start against a southpaw. If Guerrero shows he has something left, Edwin Encarnacion could move to first base with Guerrero moving into the DH slot. In fact, the Jays should think about permanently replacing Lind against all pitchers. He looked like a future star after his monster .932 OPS season in 2009, but he now has a .290 on-base percentage since and is off to .198/.283/.347 start. While he has popped 49 home runs the past two seasons (and three in 2012), you need more production from your first baseman.
If Guerrero is eventually added to the Blue Jays roster, the odd man out would probably be outfielder Ben Francisco. He and Rajai Davis both hit right-handed but Davis provides more speed and better defense. It seems unlikely the club would punt on Lind, who is making $5 million this year and $5 million next (with club options through 2016).