Well, there's a big hill. And some sheep. That's about your lot. The Union is based in a bothy, which gets a bit draughty in the winter.
Somewhat more seriously: Dundee is not a big city. It's a small to lower-medium sized one. Then again, I'm used to Glasgow, the 3rd city of the UK, so I may be slightly biased on this front.
It has all the usual high street shops you'd expect in any British city centre. You'll be fine on that front. Edinburgh is also under 45 minutes away on the train.
I'd say it's quite vibrant insofar as it is quite optimistic about its future. There's a lot of great plans floating about as to the development of the city and where its future lies, which is very interesting. There's also been a lot of inward investment in recent years, so evidently others are getting excited too.
As for seclusion, Dundee definitely feels less isolated than Aberdeen. It's close to the central belt, which is the main corridor of population in Scotland, and plenty of places are nearby - Edinburgh, as mentioned, and also Glasgow isn't that distant. It's also incredibly inexpensive to get around to such places.
Stirling really is dead, yes. Some people like that, I suppose.
If you're concerned about size: well, Dundee's bigger than Middlesborough, but what it perhaps lacks is the outlying population. While cities like Glasgow have a huge number of satellite towns, Dundee has perhaps Perth and a few smaller places like Forfar nearby which can perhaps make it less filled with amenities. I certainly didn't find it particularly lacking during my time there.
I'd strongly recommend a visit, though. Really, even if only a day-trip. You might like it, you might not. Realistically, the overall impression of a place is not something you can easily transmit in text.