I mean, if you run table(snsuser) you shouldn't get any length problem. But I'm not sure that's the result you are trying to obtain: it will be the number of occurence of each value in snsuser.
If you have a set of points that can each be described with a value x and a value y, then you have two vectors x and y that are the same length, and you can run a linear regression with lm(y ~ x), or cross-tabulate them with table(x,y).
So it all boils down to what you are trying to determine. If snsuser and fives are two vectors that do not have the same length, they are not representing the same points and you can't cross-tabulate them or perform a linear regression on them. If your goal is to understand the relationship between SNSUSER_W35 and SM10C_W35 they are the same length, and this could be correct (depends on what they really represent).