To control vine growth temporarily, you can snip the ends after a certain number of true leaves have appeared, for example, after 6 leaves. You will, after a while, get new vine growth appearing at several leaf nodes, so you haven't lost anything and possibly will get more flowers out of the operation.
To be safe and to keep Ipomoea nil happy, wait until night temperatures are above 55 to 60 degrees F. Ipomoea purpurea can tolerate a bit more cool temperatures at night. I don't know what you have growing so can't help more, but those are the 2 more popular species.
