We've not yet done a deep dive into Vivitar enlarger lenses, but here are a few handy tidbits. The timeline of Ponder & Best / Vivitar and their move from LA to CA is well known, and helps date many lenses by the packaging.
1. The well-known key that reveals the secret identity of the manufacturer doesn't work with Vivitar's enlarger lenses. They appear to be chronologically sequenced. If so . . .
2. It appears that Vivitar dropped the LU suffix for later all-silver models (therefore LU models are made earlier than 1979)
3. Post-LU all-silver models were marketed as 'Flat Field' (therefore made after 1979)
4. The catalogue numbers of both ranges are identical, suggesting they were identical apart from the branding.
5. Kino made the early 50/3.5.
6. Schneider made all the VHE's Vivitar sold from 1977-1985.
7. However, the optical components of the VHE's were all made in 1975-76 - the last of the pre-S Componons.
8. Vivitar 35/3.5 were triplets: it's printed on the packaging.
9. The six-element 'fast 50' is tantalisingly similar to the Hoya EL, but can't be conclusively identified as such. That lens came in two versions, [V2] superceding [V1]:
9a - Version 1 (pictured above) was AKA Spiratone Resomax, Jessop, Vivitar, Fujimoto, E-Lucky and Phago - all were serialled. All have straight-8 diaphragms. They date from c.1980-1984. Vivitar marketed it as part of their 'Flat Field' range.
9b - Version 2 (likely upgraded and post-1985) was AKA Spiratone Resomax, Jessop, Vivitar, Fujimoto, Durst Optar and Cambridge EL Pro Cambron - none were serialled. All have straight-6 diaphragms.
19c - I'm still finding new versions of these lenses under different names, and still not sure of the manufacturer. If you have any theories or find any variants Delta doesn't yet know, please let me know.