Single fare (with National Railcard) for the through journey
(Note that as this is a TfL+NR joint fare including Zone 1 it attracts a premium)
£4.90 Monday to Friday from 0630 to 0930 and from 1600 to 1900.
£2.45 At all other times including public holidays.
Barbican to Vauxhall is a Zone 1 only fare (as Vauxhall is in Zones 1&2) at TfL rates, so £2 peak or £1.30 off peak.
Vauxhalll to Raynes Park is a National Rail fare from Zones 2-4 therefore it is only £2.40 peak or £1.20 off peak.
Therefore it
can be cheaper to "split" however the only way to do this is to wait at Vauxhall for the OSI (
out of station interchange) to time out! Therefore, not worth the hassle!
http://www.tfl.gov.uk/tfl/tickets/faresandtickets/farefinder/current/default.aspx