Not sure how well your painted part will match the other. My impression of Brightsides product was not good on the things I used it on. Personally I prefer paint old oil
based enamel to brightsides. Not that I like either that well!
For clean edges, remove the tape before the paint sets up. Auto repair suppliers sell "fine line" tape which works great for clean edges with no paint bleeding under the tape... that's it's purpose... for great edges on autos.
I too agree.... paint the whole transom and get new vinyl port of call. Vinyl boat lettering is even on ebay... I've used them too... no difference from the "Marine lettering " that is so highly overpriced locally....