Rod Odom added 11 points including two 3-pointers for the Commodores (5-3).
Dai-Jon Parker sank a 3-pointer just inside the final minute to give Vanderbilt a 69-65 lead after Canty's 3-pointer had cut what had been a 13-point lead to 66-65. Smith sank his final two free throws and Marshall had a chance to tie it before Siakam stole the ball.
Vanderbilt's Josh Henderson suffered a knee injury at 11:15 of the first half right after back-to-back blocks by his teammates on a Herd fast break.
Siakam made 9 of 11 free throws, hitting four straight sandwiched around Pittman's 3-pointer that trimmed it to 64-62.
Jones' 3-point play a half-second before intermission gave Vanderbilt the largest lead of the first half at 35-26. He powered in a basket after halftime, then slammed in a dunk on a loose ball for a 41-30 advantage.
Odom's second 3-pointer stretched Vanderbilt's second-half lead to 47-34. Marshall's Pittman had just dunked, but only after the Herd blew a big dunk attempt and VU missed on an alley-oop.
Canty had a dunk following a turnover and scored on a three-point play for a 6-2 spurt that closed the gap to 49-41.
Smith hit four straight free throws and then made a layup for a 6-0 Marshall run that cut it to 58-54 at the 7-minute mark. Canty followed with his 3-pointer to pull within 1.
Odom hit two free throws and a 3-pointer as Vanderbilt took an early 12-6 lead. Marshall closed to 14-12 on Tyquane Goard's alley-oop dunk and one of two free throws.
But Eric McClellan scored four straight baskets, his first points of the game, including three long jumpers to widen the margin to 22-14.
The Herd pulled within 22-21 with a 7-0 run capped by Pittman's 3-pointer from the left baseline 3:46 before halftime. The Commodores answered with an 8-0 run including consecutive 3s by Kyler Fuller and Luke Kornet for a 30-23 lead.