Sekou Wiggs added 18 points and Glen Dean had 14 points and six assists for the fifth-seeded Vandals (15-17), who advance to play top-seed Utah Valley in the semifinals.
Martez Harrison led the way for UMKC (10-20) with 18 points, Nelson Kirksey tacked on 15 points and Fred Chatmon netted 12.
A tight first half ended with UMKC on top 30-29. The Kangaroos shot at a 42.3 percent clip, compared to the 34.8 percent effort turned in by Idaho. However, the Vandals remained close by sinking 5-of-12 from 3-point range.
UMKC heated up to 54.2 percent shooting in the second frame, while Idaho moved to 47.8 percent. The Vandals were able to get past the Kangaroos by knocking down 19-of-26 at the free-throw line, including 6-of-8 in the final three minutes.
UMKC lost despite outscoring Idaho in the paint (28-21) and off of turnovers (18-12). The Vandals won the battle of the boards (37-26).