Simulation says you are running something like 340hp @5000 and 400ftlb @2500. The torque curve starts at maybe 140ftlb @1000, hits 340ftlb @2000, and holds above 360 ftlb from 2200 to 4800. The vacuum problem is shown by the low torque at 1000 rpm. Comp Cams recommended rpm range for this cam is 2800 to 7000 rpm, but in reality your hp begins rolling off at 5600 rpm.
You can get better horsepower and torque numbers, with more torque and better vacuum at 1000 rpm by going to a less aggressive cam. Your idle will smooth out some, giving you better vacuum, and your top rpm will come down a little.
12-209-2 may be a good fit. 335hp @4500, 445ftlb @2500. The torque curve starts at 300ftlb at 1000, hits 400ftlb @2000, and holds above 400ftlb from 2000 to 4400 rpm. This should have much better vacuum at idle. Comp Cams shows the rpm range on this cam at 2000 to 6000 rpm. In reality, your horsepower starts rolling off at 4500 rpm, but from a 335hp peak. Idle will be "choppy to mild-rough" according to Comp Cams, instead of "rough to very rough".
Depending on where the TH350C is shifting now, shifts will move down automatically with more vacuum, but you should be prepared to change a weight in the governor to change the shift points as required.