Ok, so we have a weapon like the Ares Crusader II, which come with a gas vent system integrated. My question is can this weapon then be suppressed, or does it take up the barrel accessory slot?

From the Ares Crusader II's description:



Every other integral Gas-Vent system in a gun has the same wording of "barrel-mounted", thus it's not hard to believe another barrel-mounted accessory cannot be added.

From the start of the Firearms Accessories section:

Ta. That means that there's no way to get gas-vent recoil compensation AND a silencer unless you go with a Ingram Smartgun X or similar.

Might be something they'll change this edition. Previously, if a gun had both a Suppressor and a Gas Vent, you had the choice of one or the other when it came to firing.

Yes, when firing. But the rules in 4th ed allowed both to be put on one weapon so long as they were either both mods or one was an accessory and the other a mod. Under 4th, all things added to a weapon in its description, like gas vents or suppressors were considered mods and therefore didn't take up weapon accessory slots. They will probably stick with this when they release the combat source book but for the time being, you can either consider all such things to be accessories and therefore you don't have those slots available on the gun or you can consider them to all be integral to the weapon and the slots are still open. Ask your GM but I lean toward the latter interpretation.

(Emphasis mine)

Plus ca change,...

Haven't found any entry yet, which prevents the Ingram Smartgun X from simultaneously using the gas-vent and the suppressor.

Logically a gas vent and suppressor can not be used on the same gun, but the Ingram Smartgun has been that way for 20 years now so I think we just have to accept it.

There is a reason that the Ingram Smartgun X is the "Street Samurai's Sidekick".

Most likely comes back with the the gun modding rules in Run and Gun.
It was same in SR4 until Arsenal came out.