Along with having a pretty awesome first name, he's had an interesting career that appeared to be done by 2006. After a mostly forgettable stint with the Pirates, he came back with a couple of strong seasons in 2011 & 2012 but had a terrible 2013.
Vogelsong's numbers aren't exactly eye-popping in 2014 3.62 ERA/1.33 WHIP but he is on a pretty good stretch right now giving up 1 earned run or less and six or more strikeouts in four of his last five starts. He's not going to be dominant in every start but his ERA should stay in the slightly above 3.5 range as his opposing BABIP of .305 indicates his overall stats are close to where they will end up.
One thing that could indicate a slightly more optimistic projection of his performance is that he's done this primarily against better teams (7 of his 9 starts are against teams above .500).
Smith has spent most of his career as a platoon player. Playing with the Rockies he had okay numbers that didn't seem to be inflated by Coors field. Playing in the hitter unfriendly O.Co Coliseum did not seem to impact things.
Signing with the Padres in the off-season made a minor impact at best. He looked to continue being a decent but unspectacular season. In his first 41 games as a Padre he's been an absolute monster .338/.419/.615 200 OPS+ 6 home runs.
Petco Park seems to hate most hitters but for whatever reason it has been good to Seth Smith so far (1.275 OPS) compared to .728 OPS away from Petco which is closer to what was expected when the season began. That may be a good sign for the future but not his .380 BABIP.
Best case scenario would be batting average going from .338 (current) to the .270-.280 range. In his career, he has crushed right hand pitching .859 OPS and has struggled against lefties .594. Based on his splits in 2014, the Padres are utilizing him VERY smartly by rarely letting him hit against lefties. This may allow for his overall numbers to stay solid but this would limit playing time in the long term which would mean less counting stats such as home runs.
His most similar player through age 30 is Eric Byrnes. In his age 31 season he had a decent season .286/.353/.460 21 Homers & 103 OPS+ while respectable, inexplicably earned him 11th place in the NL MVP voting. After that he played (poorly) in parts of three additional seasons before his career came to an end.
In one corner you have someone who is coming off a poor season and seems to have bounced back towards a middle of the pack pitcher. In the other corner, you have a player off to a scorching hot start that is utilized very efficiently by his team (which has a negative impact on plate appearances) and is likely to be more valuable to his team than fantasy owners.
Cage Match Winner: