I made broccoli butternut squash soup in vitamix with whole broccoli. HORRIBLE! Should I have used just the broccoli florets? Worst taste EVER to to

Did you use stock? Or just water? You need to have some stock element even a good veggie stock. Also seasoning. MSG is not horrible...so use some 'accent' with salt, that will help wake up a flat stock or soup. Or use a tsp or so of fish-sauce, dark miso, or soy sauce. When ever a soup tastes flat, it's usually under seasoned or uses weak stock.

I find the stems to be sweeter than the florets. But I've always peeled them before using them. I make broccoli soup with just the stems as a matter of course.

I don’t know why the broccoli stems would be a problem as I’ve been eating them since I was a little kid.

I still cook these and they are just as tasty as the florets although I often peel the skin off (it’s a bit tough).

Broccoli stems are the typical base for broccoli soup, whether it be from scratch or in a commercial product (e.g. canned soup).

Perhaps something else contributed to the off flavor.

I use broccoli stems in soup often, but I peel them first. The skin can be tough and on the bitter side.

Could be that broccoli stems and/or other factors that contributed to the bad taste.
If you could send us a link to the recipe, or tell us key ingredients & steps, someone here could provide some analysis & suggestions for next time...or whether the recipe just sounds off.