> Beyond that, it's 0.8-1.2g of protein are recommended MINIMUM daily intake to avoid diseases originating from protein deficiencies.
Eating below the minimum RDA of 0.8 g/kg is unlikely to cause diseases of protein deficiency. The RDA minimum is fairly generous to allow for a certain margin of error. Most of the studies cited by https://www.ncbi.nlm.nih.gov/books/NBK234922/ indicate that 0.6 g/kg is likely a sufficient reference, and that a recommendation of 0.75 g/kg allows for enough confidence to cover individual variation. There was one study cited which found a 0.8 g/kg minimum, but most of the rest found that lower values should be sufficient; it sounds like 0.8 was picked just to be on the safe side.
> At 220# that means 80-120g protein as a minimum. That's a fair amount, and far more than you're likely to get from vegetarian sources at the time alone.
I doubt that most Roman soldiers were 220#. The minimum height requirement was 5'5", and estimates are that the average height was 5'7".
> There's also a minimum amount of essential fatty acids to consider... at the time it's not like they had canola oil.
There's a certain vegetable oil that Rome and Italy are quite well known for.
I'm with you on being skeptical that they were vegetarian, but I think you're overstating the requirements a little bit. I think it's fairly likely that they ate a fairly low meat diet just due to the logistical difficulties of obtaining fresh meat or carrying preserved meat.