don't really get this 'vote snp and you get the tories'

It's simple arithmetic. If the polls are right and the SNP get all those Scottish Labour seats, Cameron gets the largest number of seats and, as the incumbent, get the first and best shout. If Labour try to form a government with considerably fewer seats than the Tories (it's my guess that they won't even get a sniff at that in any case), they'll collapse within weeks, SNP or no. The SNP is very strong at the moment and will not want to be seen to be propping up a lame duck. So, unless the polls shift somewhat radically, a great big SNP presence means, at best, another election sharpish or, at worst, another five years of Toryism. Take yer pick. But vote Labour!