Air carrier Captain reported an NMAC with a light aircraft on descent into PVU.

Narrative

On arrival in to PVU to avoid GA traffic SLC approach took us off our filed routing for multiple training aircraft holding over the FFU VOR and attempted to vector us around the mountain that FFU VOR sits on. We were given a heading of 040 and maintain 10;000. After this instruction brought us over the lake; SLC approach gave us a turn to heading 350 and a descent to 9000. As we began the descent to 9000 we immediately got a TCAS TA followed immediately by a monitor VS RA. We responded to the TCAS RA according to SOP; let ATC know that we were responding; and advised when clear of conflict. After we were clear of conflict we advised ATC that we would like to file a NMAC report. A contributing factor in this TCAS RA was that the other aircraft involved was not adhering to GA altitudes for the area and ATC thought they were at 8500 when they were at 9000. FFU VOR sits outside of the PVU Class Delta and is regularly used by high volumes of training aircraft to practice holding at altitudes of 8000-9000 feet while not required to talk to SLC Approach. My suggestions would be to incorporate the airspace above FFU into the Bravo shelf for SLC that ends 1.5 miles before the VOR and drop the Bravo floor of that sector to 8000.