Will putting telephone pair through a DPDT switch affect ADSL / VDSL?
Any impedance "bumps" in the circuit will have "some" affect on achievable data rate, but as long as a switch is a sensible one and the connections are competent and the pairs are kept as undisturbed as possible, I'd expect no significant impact.

Keep twisted pairs twisted for as much of their length as is sensibly possible - no need to overdo it. ie Avoid having wires form a large loop or the pair broken up so wires go via different routes. 

I dislike crimp connectors of any sort for long term use. They may very well work indefinitely, but I try to solder connections where possible - with memories of bad experiences long ago. YMMV :-).