Mid boom sheeting does not provide any 'extra' vanging capability over end-of boom sheeting. Either type, when sheeted in hard, will make the vang a bit unnecessary under typical conditions, but where the vang comes into play is once you've eased the sheets and start to reach and run, at which point the downward pull of the mainsheet is increasingly reduced. Now the vang is required to control the boom lift and twist.
IMO a vang is as necessary a bit of gear as most anything else and even a fixed strop of the right length would be an improvement over no vang at all. It's also generally a fairly inexpensive bit of tackle to add on for a basic vang, but you can spend some coin on the various rigid vangs available.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)