Grant
I think you will look fine with a waistcoat. It would'nt be my first choice, however if you do wear the waistcoat only then leave the bottom button undone.

I may be able to bring an extra argylle for you, as my local tartan shop has a few on consingnment and I was going to clear them out, give my your size and I will see if they have anything, the prices I have seen are between $40.00 and $80.00 CDN.