Like others, I've also used both - vetbed first then switched to fleece. There are pros and cons to both, as already mentioned.
I agree that hay (especially seeds) gets stuck into the pile of vetbed - I found it hard work (and impossible) to get everything out of the vetbed prior to washing, even after beating and hoovering it, but it is extra comfy for the piggies. If you're looking at the official genuine Vetbed (by Petlife), then the Vetbed Gold also has the added protection of an antibacterial/fungal substance built into the fibres. I used this and it was great but I now use fleece as I find it much easier and lower maintenance, although I also find that hairs really stick to it.
All things considered, I'm glad I switched. Compared to fleece, I'd say the genuine Vetbed is a bit more effective at wicking away moisture and keeping piggies drier for longer but as long as you have something absorbent under fleece and change it often enough you'd never really notice any difference. I also sometimes put a piece of Vetbed in their "bedrooms" for extra cosiness and the best of both worlds, especially if they're ill. I also like treating them to nicely patterned fleeces (whereas their Vetbed was plain and boring) although that's more for me than them!