I wouldn't be without a tent carpet. Just finishes the whole thing off. The floor of our tent is black so it wold be much hotter without the grey carpet on a sunny day. Also much cosier when the weather is less great. I'm precious about ours (pale grey & not machine washable, silly really) so I have washable doormats from dunelm too.
You don't necessarily have to have the exact carpet for your tent, I'm not sure about Vango but in our Outwell there are no corresponding fastenings or similar which mean you can only use a certain carpet although I'm anal and insist on matchy-matchy. Friends have a Kampa and have just cut any old tent carpet (that they found on ebay) down and hemmed the edges to fit. You'd never know. Having a perfectly fitting one designed for your tent saves hassle though.
As far as footprint goes, I think a big part of it depends how precious you are about your tent. We never bothered with our old Hi-Gear, the sewn-in groundsheet was good quality, never leaked or ripped and didn't get damaged as we only ever pitched it on level grass. When it came to strike the tent, I just gave the outside of the groundsheet a rub down with a towel if it was wet or grubby and that would suffice.
With our larger new Outwell, while the groundsheet quality is much better than the Hi-Gear, I'm way more precious about it (noticing a theme?) and conscious it's supposed to last us a long time. We bought the footprint because it was a piss in the ocean in the context of the price of the tent. It's a much larger tent and the fact the groundsheet stays clean and dry because of the footprint saves time when packing away. It's also helpful in demarcating the position of the tent when pitching which helps ensure an even pitch - useful for a larger tent.
Never had a groundsheet. However, a bit like ViviPru when we got our 'mega-this better last a lifetime' tent, we did put a bit of tarp down on a site which was a bit rough and stoney. Carpets are a different matter. Wouldn't be without one but as long as it roughly fits I'm happy. Our Outwell carpet sort of fits our DeWaard and we just kept it rather than buy another carpet. Plus the quality of the Outwell ones are fab. Hardwearing rubber back so check out the quality before you buy. We just put picnic mats down in bedroom areas...easy to shake out at the end of the trip.
Agree with others on the carpet, we have an Outwell but I didn't buy the correct carpet for the model (it was in the sale), it nearly fits and does the job. Makes a big difference. Haven't got the footprint though.
If you could only afford one I would say footprint every time, it keeps the under neither of the tent cleaner, so easier for packing away, also extra protection against stones,sharp objects etc. Carpet makes it luxury, but picnic rugs etc can be used if needed.