Alex, most trucks have the rear belt drive the alternator, the middle one the P/S pump,the front one drives the A/C compressor, and the vacuum pump is driven with a very short belt off the alternator (alt has a two-groove pulley). If it's a non-A/C truck they kept the rear and middle belts the same, but they added a two-groove pulley to the P/S pump and ran the front belt to it too (so you have two belts that are the same now), and vacuum pump is still driven off the alternator. So that's an unusual setup you got there, but it is also the same that I came up with (all on my own too!) for my truck - I have the short alternator belt that don't go around the crank pulley so every time I revved the engine high the P/S and A/C belts would scream bloody murder, I figured I need a belt on the passenger side that goes around both the crank pulley and the water pump pulley and the only thing I could drive with it was the vacuum pump, so I ditched the A/C compressor (dead anyways) and ran the middle belt like yours is to the vacuum pump directly, then since my P/S pump only has a one-groove pulley I moved the whole pump along with its brackets 3/4" forward of it's factory location so it now lines up with the front belt (normally used by the A/C compressor). Works great, all belts stay nice and tight and deathly quiet even when running against the IP governor.
