Originally Posted by
tdracer
Not necessarily - at least if the vapor is 'designed for'. The early Saturn V's had so-called 'ullage motors' to get the fuel to the tank fuel outlets for staging and in-flight restarts. However they gradually discovered the ullage motors were not really needed and got rid of them.
You are referring to the second stage which was full of fuel at the point of ignition. The S-IVB stage needed ullage for the TLI re-light, using the APS motors. Re-lighting Musk's rather large rocket with only a few percent fuel will require a means of getting what little fuel there is to the bottom of the rather cavernous tanks.
S-IVB - Wikipedia