This doesn't sound right. But if it is, then I suppose you could order a second land line from Verizon and ask them to put your existing number on the new line and to assign you a new number for the line carrying your DSL. Then port your existing number from the new line to Vonage which will have the effect of cancelling the new line. Then have Verizon cancel the other number and make your DSL stand-alone.
(This will probably cost you some installation fees but they will be less than the cost of the psychiatric care that you'll need if you continue to try to make sense of Verizon and Vonage's policies.)
Alternatively, you could cancel your DSL service so that Vonage can order the porting of your number, then re-activate your DSL as stand-alone service after the dust settles. This might leave you without DSL for a day or two, of course.
Or, ask Verizon to set up "ring-mate" selective ring on your existing line and give you a new number as the primary ringing number and make your existing number the second ring. Then have Vonage port the second ring number and, once that is complete, have Verizon terminate your wireline voice service and convert the line to stand-alone DSL.
None of these suggestions is particularly elegant -- but I doubt that you can do anything to change the policies and procedures of Verizon (entrenched bureaucracy) or Vonage (administrative incompetence.)