Sure, why not? I am not a vinyl siding expert, but when I have helped, we often trimmed down pieces as needed. If it works and looks good then that is all you need.

A cutoff edge of siding won't be as stiff as the formed metal starter strip, but with enough nailing it ought to work.
Casey

Thanks, I figured it would work ok.

EG

Lowes has vinyl and metal starter strips.

You just did not ask nor look closely enough.

Your vinyl siding manufacturer also makes starter strip which is mandatory for use with the manufacturer's siding products.

Use of anything other than the manufacturer's approved starter strip can void any warranty the manufacturer may offer on its siding products.

Use the right starter strip or risk paying out-of-pocket for your own mistakes in the event of a product failure.

I've used a strip of vinyl siding, a 2 inch piece cut from the bottom of the siding to finish the strip run, it works but I would recommend that you nail it every 6 inches to secure it properly, it's a tight fit but at the cost of vinyl siding a piece, it's not economical.