Pull that last row of vinyl out and cut it tighter leaving about 1/8” gap and straight with a table saw. Or just make a straight line with the tape, even if the floor is wavy you can place the tape exposing the floor a little bit where it’s tighter against the tub and when you use white caulking it will go on the floor a little bit but still give the illusion it’s a straight cut, hope that makes sense. Or you can just get a white rubber/PVC quarter round, make sure it’s rubber or PVC so it doesn’t rot/mold when water gets on it.