Removing a Key From HSTORE in Postgresql
You're trying to use the expression item.details::hstore - 'vin'::TEXT as a PL/PgSQL statement.

Where's the result supposed to go?

At a guess, I think what you intended to write is:

item.details := item.details - 'vin'::TEXT;


i.e. "set item.details to the old value of item.details with the key vin removed".

It's kind of hard to be sure without more context.