I'm not sure what's going on, really. Normally a vendor will have everything that you've sold it in their inventory unless you don't enter the same cell for the normal respawn time, which is three game days; though in Fallout 3/NV/etc they've taken to putting the vendors' chests in a separate cell presumably to work around their goods never being "restocked". That's not the case with The Sink's vendor, though, with the chest being in the same cell (hopefully that makes at least
some sense!)
I've just taken a look and spotted a suspicious-looking script that's attached to the chest:
SCN NVDLC03SinkVendorContainerSCRIPT; Vendors don't seem to use caps in owned containers, so this script resets the Sink vendor's caps when the container resets.; TLS 03/29/2011BEGIN OnReset If (NVDLC03SinkMainComputerTalkerREF.GetItemCount Caps001 < 10000) NVDLC03SinkMainComputerTalkerREF.RemoveItem Caps001 10000; NVDLC03SinkMainComputerTalkerREF.AddItem Caps001 10000; EndIfEND
In itself it's rather inconclusive, but I have to wonder if it's the culprit since it's mucking about with the vendor's available number of caps.
I've conceded defeat in terms of finding the problem and decided it was easier to create a new vendor who doesn't seem to be afflicted with the same thing...