Vauxhall has the same trains as London Waterloo (same line 1 stn along), plus easy sight of passing Eurostars.

Unless it's the Eurostars you are after, much better to go just along to Clapham Jct as you get much Victoria traffic and 1 or 2 from London Bridge.

i found vauxhall station quite good for taking photos of eurostars from waterloo as they are not travelling that fast there.Also plenty of southern units through there too and trains from there to 2 london termini(victoria and waterloo).I usually go from one to vauxhall then back to the other.