Yeah, the light armor is probably rather pointless regardless of that but it would feel a little better than sending in guys without any armor - perhaps simply change the basic jumpsuit sprite to include some sort of "basic vest" which would explain the tiny armor that the basic soldier get.
When I first designed them I also had in mind adding some weapons, along the lines of aliens getting some sort of laser / pulse laser weapon for early encounters. Still strong guns, but the ballistic armor would have a point then (until the aliens upgraded weapons).
Another idea in that vein is to play around with armor vurnabilities / strengths (I think UFO: Aftermath did this a little). Not much point as long as the aliens are only packing plasma (and explosives), but with more weapon and damage type we could have armor that is engineered towards giving high protection against a specific damage type, versus an armor "line" that gives decent protection against most weapon types.
If you don't know what you're up against you pick the decent one, but if you know what you'll be facing (if it would be possible to have various aliens favour different weapons) you could equip the niché stuff.
Granted, that could be needless complexity - but it could probably be fun the other way around. Picking the right gun for the job and not merely make it a "Heavy Plasma for everyone" kinda deal. That's not this thread though
How's this:
Medium Ballistic Vest Type III
Front 25 (+5)
Right 15 (+3)
Left 15 (+3)
Rear 15
Under 0
Weight: 10
Heavy Ballistic Vest Type IV
Front 55 (+20)
Right 25 (+10)
Left 25 (+10)
Rear 15
Under 0
Weight: 20 (Possibly 25, or even 30 - to keep it from seeing mass-deployment - A rifle and some mags should basically be it for anyone wearing this)
Light Personal Armour
Front 35 (+10)
Right 20
Left 20
Rear 15
Under 10
Weight: 7
Alien Alloys Required 10
Medium Personal Armour
Front 50
Right 40
Left 40
Rear 30
Under 15
Weight: 12
Alien Alloys Required 25
This way the Heavy Ballistic Vest is *mostly* inferior to the Standard Personal Armour (short of the front armor), yet you might still want to pack it for a few "door busters" (and of course, it would be a lot cheaper) until the Heavy Personal Armour becomes available.
The Light Personal Armour is ideal for the riff-raff, as it is better than the Type III ballistic vest yet a lot lighter than the Heavy Ballistic vest.