Issue with stalling when in gear

With a 6 have less cyls than an 8, one [ or more ] cyl going out is going to be even more noticeable....
My comments were based on the driver having enough nous to feel the difference between all 6 cyls 'working' versus 1 or more not working.
You wouldn't need a vac gauge to tell you that & it is not going to help you find the problem.
You would just be wasting time hooking up the VG....to tell you what is already evident.