I just sold one for a Monza without AC and we pretty much determined it will fit a Vega. The difference is the length of the tubes with a Monza being a little longer because it is reversed in the heater box for clearance purposes with a V8. Unfortunately Monza heater cores without AC are practically impossible to find too. I have a few extra new ones that I got for myself and as I mentioned just let one go, I'd have to check and see what I actually have left.
I may actually still be able to get a correct Vega one but they are expensive. If you need one feel free to contact me and I will see what I can do. If anyone else is interested let me know, maybe I can get a bulk deal, I'd be glad to investigate............
cjbiagi@yahoo.com