Among their claims is that proposition B will not take away funding for CalTrain electrification.
This is not technically a lie. But it is the case that the VTA's monomaniacal obsession with BART is leading them to steal promised funding from any and all sources, including $$ promised from the 2000 Measure A sales tax for CalTrain electrification.
Defeating B is the best way to force them to abandon BART, build something they can actually afford (just run ACE trains back and forth to Fremont all day instead of letting them sit around Diridon Station !@#!) and keep their other promises.
For much more complete arguments see VTAWatch.blogspot.com.