ZSU-57 would require a major redraw of the hull and running gear, four road wheels vice five on the T-54. IIRC the dimensions are slightly different too. Probably a whole new vehicle when it comes down to it. That said, I’d love to see one. Fielding it would be interesting. They did feature in the 1972 Easter Offensive.
The bridge layer, would similarly be a mighty beast but I can’t imagine too many folk wanting one for gaming purposes. I could find a use for the BMR mine clearing version(s) for my Afghan war Soviets but I suspect I would be almost alone in that desire.
Some items that might find appeal are some Vietnamese soft skins, GAZ-69 field car, GAZ-51 2.5 ton truck or the ubiquitous Zil-131 and its Chinese clone. Good for playing truck hunting games on the Ho Chi Minh trail and useful for other post-war conflicts.
Some light AAA pieces would be nice. ZU-2, ZU-4. There was even a Vietnamese of the BTR-50 that had a shielded ZU-2 on the rear deck. The Squadron book on Vietnam armour has an illustration.
For me, I’d love to see some Chinese vehicles, the T-63 and T-62 light tanks. They didn’t feature heavily in the Vietnam War proper but did see heavy service against Vietnam in 1979. Of course someone would have to be convinced to produce a 1970s Chinese Army and I don’t fancy my chances on that front.