If you are looking for a butter / margerene replacement, finding something with no diary can be a nightmare as most have milk or milk by products in them. When I had to avoid dairy, Vitaquell was the only completely dairy free margarine I could find. They do (or at least did) a range of products, one with a better omega 3mega 6 ratio
I find Bovril is very good for welding the cap to the jar, meaning I only benfit from half of the contents. I have never tried Vegemite, but I believe you are not alone in your observation.