The 109th-ranked Gulbis, who also won the tournament in 2010, is the first qualifier to reach a final this season. The previous qualifier to win an ATP World Tour title was Jarkko Nieminen last year in Sydney.
Gulbis also became the third qualifier to win the title at Delray Beach, joining Kei Nishikori (2008) and Andrew Ilie (1998). The Latvian is set to move way up when the next rankings are released.
"I feel really good about it and I just talked to my coach (Gunther Bresnik) and he's happy, I'm happy, everybody is happy," said Gulbis, who has been ranked as high as No. 21 in February 2011. "We didn't think that it was going to happen so fast.
"My coach thought I wouldn't go back into the top 100 until around April."
Gulbis improved to 3-0 in ATP finals. He also won in Los Angeles in 2011.
It was the first time since 2007 in Houston that two players ranked outside of the top 100 reached the final. The previous player to win a title while ranked outside the top 100 was then-No. 141 Ricardas Berankis in 2012 in Los Angeles.
Roger-Vasselin, ranked No. 105, made his first appearance in an ATP final.
The Frenchman was serving at 5-4 in the first set when he had his one set point in the match. Gulbis saw Roger-Vasselin pause for a wind gust and started a conversation with umpire Fergus Murphy just as he went to serve again.
Gulbis saved the point with a scorching backhand down the line and went on to the tying break.
"Yeah," Roger-Vasselin said when asked if Gulbis' action annoyed him. "But honestly, he played good, he was just too good.
"It's always tough to lose in the final. But it's a great week for me."
Gulbis admitted to trying to shake Roger-Vasselin at that critical stage when the Frenchman had set point.
"I cannot say that sometimes I don't do it on purpose because I know that the guy is nervous," Gulbis said. "That's part of the mental game. If you cannot be tough enough to deal with my talking to the umpire then, 'OK, you lose the set.' "
Gulbis took control of the second set when he broke Roger-Vasselin's serve in the seventh game when the Frenchman netted a backhand half-volley.
Gulbis won $78,000 and Roger-Vasselin took hom $41,000.