As I mentioned in another thread the Vauxhall NR reader do not seem to
update the status of the card as you pass through. I often take
Vauxhall NR - Waterloo - Bank - Shadwell DLR (SWT and the Drain is
usually quicker than underground via Oxford Circus) but if I use the
Oyster to enter at Vauxhall NR then use the validators on exit at
Shadwell DLR the screens at Shadwell show 'Enter' rather than 'Exit'
and I get an unresolved journey, even though it is all within my
Travelcard zones. Vauxhall NR also does not show up on the journey
record on the large ticket machines - although the DLR also shows up
as blanks come to think of it.

You have to validate in at Waterloo (Waterloo & City) platforms, as
you always need to have swiped in and out for the part of your journey
where Prepay is valid, even if you havea vaild travelcard covering all
the zones.