nmtop40 wrote:
Is it Eurostar or Waterloo BR?

If the latter, you can go from Victoria to Vauxhall on the
Victoria Line (do mainline trains go from Victoria to
Vauxhall?), then back from Vauxhall to Waterloo.

Yes, in fact if your going from somewhere on the Southern to Euston or
King's Cross and want to avoind the Waterloo cruch it makes an alternative
interchange.