Accounting by mistake added a value to the "REPRESENTS SUBSIDIARY" field to few Vendor records; however, once it is saved, the field is not available to be removed. • Is there a way to remove this?

No that cannot be removed You'd have to make a new vendor record and inactivate the old one

Sometimes Netsuite makes things too complicated. We just had a similar situation with a new Location. We selected the wrong subsidiary and we noticed that as soon as it was saved and we could not change it. I would understand if the record had transactions already liked to it, but no when there are not child transactions.

There are important things linked to it that occur in background. That's why certain things get locked like that.