My college English teacher explained that you all means 'all of you', that is more than one you.
"las vacaciónes" was rejected in favor of "sus vacaciones" But the coaching statement did not seem to be helpful. It stated: "vacaciones" is plural, not feminine. However, it is both plural and feminine. The real problem seems to be the use of the definite article "las" instead of the possessive "sus".
Sounds like that coaching statement was the problem. An error. 'sus vacaciones' is simply 'your vacation'. As opposed to THE vacation. Las vacaciones.