Your basic problem is that you are trying to add the pistol to the base form ID, not to the actual chest that Chet uses in-game. You need to add the item to a specific chest, not to a type of chest.
The actual chest that Chet uses is in the interior cell named NVVendorChestsCell. Chet's chest in-game doesn't have a REF ID (they left it blank), so you'll have to modify it to give it one. In other words, go to NVVendorChestsCell, find Chet's chest, change its REF ID from blank to something like VendorContainerChetREF, then use that reference in your script.
Alternately, if you are worried about mod compatibility, you can create a new chest that is owned by Chet and place your items in that. The trick here is that vendor references (like Chet) can only have one Merchant Container attached to their reference, and Chet's reference already uses the chest in NVVendorChestsCell. This means that your new container will need to be in the same cell where Chet is located, otherwise it won't be used. Also, make sure that you set the new container's ownership to GSChet (Chet's base form ID).
Also note that the 10mm pistol that you add will have a health of 100 percent. If you want the pistol to have variable health condition, you'll have to use a leveled list. See the CondLeatherArmor in Chet's chest for an example of how to do it. It's pretty simple.