The Primera Division club had dropped points from draws with Lyon and Rijeka but, although their performance in the 1-0 win was far from convincing, they held firm against a Guimaraes side looking to stay top of the group.
Fernando Russi came close to opening the scoring for the Portuguese outfit with a close-range shot in the eight minute and Nosa Igiebor was unlucky to miss the mark for Betis soon after.
The first half lacked an obvious protagonist but Jorge Molina Vidal drew gasps from the Estadio Benito Villamarin crowd when he drilled at Renato Douglas from the edge of the area.
The visiting goalkeeper defused the shot to ensure the game remained goalless at the break.
Five minutes after the restart, however, he was picking the ball out of the net as Vadillo stole onto Molina's low cross from the left to tuck home.
Chuli sent Douglas sprawling again with a bullet header but Stephan Andersen would soon be the busier keeper as Guimaraes set up camp around their opponents' box.
Russi's header was tipped round a post before Andersen parried his sighter from a Tiago Rodrigues cross.
Nii Plange found space to test the Dane again but this time Lorenzo Reyes stepped across to block the shot.
Marco Andre Matias then swept up the rebound, only to stick his finish over the crossbar.
From that point forward the game degenerated into a scrappy exchange of fouls and the whistle blew just after Nono had steered a tame free-kick wide of the Guimaraes goal.