Can I use vegetable oil spread (blue bonnet) in place of butter for a crumble topping

Yes, you can use margerine to make a crumble, but there are probably healthier options than BB.
If that is what you have on hand use it and do as previous post said-keep it cold, work quickly and enjoy the crumble.

If that's your only choice than I would make sure the spread is very cold before adding it and be careful not to overwork the crumble as you're making it.

Are you in a rush? Last minute discovery that you didn't have the ingredient you needed? Then yes, by all means, go ahead. It won't be the same and, in my opinion, butter is ALWAYS better, but sometimes you just need a fat to mix with your solids and you've gotta grab what you've got. If you are substituting for other reasons (e.g., you prefer a vegan solution) there are much better alternatives (in terms of taste and texture) such as coconut oil.

I have used margarine, it turns out fine. Not sure about the soft spreads, though.

I would think you'd get quite different results in terms of taste and texture, but I've never tried it myself.