Unless it is not physically possible I would change out the line set. her have changed about 6 line sets this year that were leaking. Going from 22 to 410 increases that likelihood. Hover if you are going to keep the line set then have the contractor flush with RX-11.
As far as testing for leaks. Always use nitrogen pressure. Pulling a vacuum to test for leaks is a waste of the. no matter how good a vacuum you pull you are not getting more than a 15 psi differential. the absence of pressure in the copper and the atmospheric pressure outside. (14.696 psia) Using nitrogen you can easily apply more than ten times that. just my 2 cents