Actually, Bhoins, not all rulesets permit it.
Under MT, Vacc 0 is the marine default. higher levels are NOT readily available. Vacc Suit serves as battldress at -1; level 0's are not permitted to serve as.
Likewise, ships troops have a 1/6 chance of a battledress reciept if HW is AvgStellar+, on one of three tables, and a 13/18 chance per year of getting a reciept to spend. Likewise, Infatry recieve vacc suit at the same rate. This is an expected 13/324= 4% chance per year of reciept of battledress skill when ship's troops, and 4% chance per year of infantry recieving vacc suit. Commando school includes a 33.333% chance of Vacc suit.
So, extrapolating form the numbers, that means a
years in Infantry % chance of vacc suit 1+
1 y= 4% (1 FT per company)
2 y= 8% (1 Sq per company)
3y= 12%
4y= 15% (1 Sctn per company)
5y= 18%
6y=22% (1 Plt per large company)
7y=25%
8y= 28%
9y = 31%
10y = 34% (1 Plt per Small Co)
11y = 36
12y = 39%
13y = 41%
14y = 44%
15y = 46%
16y = 48%
17y = 50%
18y = 52%
19y = 54%
20y = 56%
Only 56% of 20year infantry will have Vacc suit 1+... by a strictly random CG... still, even if we only pick from the one tables with relevant skills, the base line is a mere 13/108=12% per year.
the mechanics do not support the assertions of Mr. Wiseman as quoth so often. Heck, they don't support the Vaccum worlds having vacc suit infantry in any numbers!
Clearly one or both are broken... but T4 is not much better, and was written with a knowledge that it would be picked apart by canonistas...
Personally, I discount it on sheer cost alone, as the hardware (We'll assume a suit has a 1/2 career lifespan for this, or 10 years) costs are about MCr 1 per soldier (assuming a 50% attrition per term) so equipped for field duty. Plus weapons. Combat armor is one 5th the cost, and probably a slightly higher lifespan, but we'll assume the same ten years. 5:1 ratio in cost, militarily, often equates to a 20:1 numerical advantage in deployment....
And since all marines can wear COMBAT ARMOR, without question (it uses Vacc Suit skill explicitly in the editions which qualify if), Combat Armor provides the same protection (AV and TH tables MT/CT), and the same LS Endurance...
Likewise, the Battledress is NOT JUST AN ARMORED VACC SUIT. (That's combat armor.) Battledress includes strength augmentation, autowalking, and load reduction systems. It will, by very nature, be harder to operate, more dangerous to the wearer should it fail, more dangerous to bystanders and equipment, and more maintenance intensive. Not to mention considerably bulkier.
IMTU, the Imperial Marine infantry and ST combatants generally are issued Combat armor, not BD. True, it's chameleoline, commed-out, integral computer and hud, IR Suppressed, but it also is capable of being worn by VaccSuit 0 troops.