Indeed, the original Fallout had quite a bit more in the way of stats for armors and damage types; Might have been some leftover heritage from the early initial attempts to use the (more detailed pen & paper) GURPS generic roleplaying rules system, before the decision was made to go SPECIAL instead. Fallout 3 and later games have simplified many things. F3 did not even have any DT to begin with - In F3 you could simply plink powerarmored enclave troopers to death with a simple kids toy BB gun. Let's be thankful that FNV at least does have a DT system in place, even though it is not necessarily ideal in execution or even internally consistent in many cases..
I would find a simplified one DT value vs all damage types quite acceptable for gameplay streamlining purposes; that is, IF these DT values would be internally consistent and plausible when compared to each other as well as the armor types explanation/description in existing Fallout lore.
To renovate/recalculate the FNV DT values, it would be necessary to at first come up with some sort of mathematical formula and variables to factor in percentages of surface area covered vs percentage of area not covered or with weaker coverage than the main armor piece like a vest, as well as armor materials abstracted/averaged out generic toughness (per thickness and/or weight over surface area units) versus the most common damage types.. Bullet/fragments, blunt or edged/pointy objects damage types would be more common and IMHO thus more important on the averaged out armor material toughness rating than frex lasers or plasma. This DT recalc formula could be reasonably detailed, as it would not overtly complicate the game - all it would do is to calculate very simple end result DT values, which could in theory then be simply plugged into a fairly straightforward armor rebalance mod for FNV/TTW.
I have no doubt that several FNV armor rebalance mods do exist and/or are under development, but have any of them so far taken this kind of a bit more systematic approach to the issue, like coming up with a formula or some tables/spreadsheets based on both realworld numbers as well as Fallout lore first, before using those tools to assign internally consistent and plausible DT values to the armors and outfits?
