I think it's easier done the other way around. :)
I have a Synology DS1812+ NAS and my APC SmartUPS 1500 is plugged directly into the NAS via USB.
I can see basic status of the UPS (i.e. on/off grid power, approx minutes remaining if on battery) in the NAS control panel, and the NAS will act as the UPS network server itself. The documentation isn't terribly clear on that point (it reads like you need an external UPS network server), but it works. Hopefully your NAS is similar in this regard?
Then you can run the free WinNUT utility on your Windows box(es), to listen for a warning from the NAS (via network) about critical power.
When the UPS is reaching critical battery level, it signals the NAS and the NAS instructs the PC to shutdown, before shutting itself down. My NAS will start itself up again when the UPS power levels return above critical, but my PC needs starting manually (I haven't investigated WOL - wake on lan - but it might be feasible).
Here is the latest version of WinNUT that I can find, which includes Win7 support so should work with Vista:
And I found this blog about configuring a Synology NAS and WinNUT with an APC UPS - hopefully with your NAS and UPS, it'll be a similar process...