Well, you should allow for Reflec as well, as that is specifically designed to go under cloth and clothing.
I'd allow for something like a flak jacket over most other types of armour, including vacc suits, though it may need to be specially designed to do so. For example, a standard military flak jacket can't just be added to a CEV suit, but one designed as an armour upgrade for that suit could be. Military concerns are not the only consideration and I would expect most civilian vacc suits would have some option for an external durability cover. Even Battledress could have high-threat external plating added (in this case more likely to be directly attached rather than as a jacket). Tank crews have been adding extra armour (improvised or otherwise) ever since tanks were invented.
Also keep in mind that high tech vacc suits *are* in effect clothing and would definitely not prevent having a flak jacket being added.
However, I'd question if cloth could usefully be added under a vacc suit, as I would think it is going to interfere with the life support functions. It would certainly cause issues with an autodoc. Even "clothing" vacc suits would be put on with no or specially designed underwear so that the waste management functions could work.
Rule of thumb: if the armour is designed to fit under or over whatever the character is wearing (clothing, vacc suit etc), then you should allow them all to be combined. In particular, I have no issue with Cloth/Flak plus Reflec.
However, once the *mass* starts to add up, you may want to assess a possible initiative or exhaustion penalty.