That makes no sense whatsoever. If you elect an MP who isn't a Tory it prevents the Tories forming a government regardless of the party so if your only intention is to prevent a Tory government then it really doesn't matter whether you vote Lib Dem or Labour.
You may be right in claiming that electing a Labour MP will mean that it is slightly more likely that there will be a Labour government rather than a Tory government. However, unless there is a massive turn-around in popular opinion then they have absolutely no chance of forming a government anyway.
In any case, if the polls materialise into votes then the most likely outcome is a hung parliament in which case it will be the Lib Dems who become the most important party in Parliament. Perhaps we might even get a democratic electoral system! One can always hope...
