Have you actually been tying them to the cane as they grew? They wont grow up them without your doing so. If they are already grown and sprawling you are just going to have to put up with that for this year.

Or are you sure they are not the Tumbling variety,as they are meant to trail over the pots?

As Moon_Grower says, tomato "vines" are actually floppy bushes, and need to be tied to their supports, preferably with something soft, so it won't cut into the stems as it grows. Older stems can be brittle, so it's easy to break them by tying them up now.

As a novice tomato grower I didn't tie my tomatoes up to their canes very well and they started to flop over. My husband showed me how to tie string from the roof beams of the poly tunnel and this got twisted around the stems of the tomatoes and knotted into place. They are now standing tall as they are getting support from the secure string and I can keep twisting the string around the tops as they grow.

Exactly Roz