Same here, happnening to me too. Also occurs exporting older client packages.
Looks like a proper bug to me! I find it quite amusing noone raised it earlier (other than the below thread that is):
https://www-secure.symantec.com/connect/forums/installer-packages-are-missing-antivirus-signatures
I guess in the meantime, we can worksround it by copying the full defs from "<Program Files>\Symantec\Symantec Endpoint Protection Manager\Inetpub\content" into the client installer folder and renaming as vdefs.zip.
Note: For the correct defs, look inside the ContentInfo.txt file in the above location, as it will list which GUID corresponds to which defs (i.e. 32bit and 64bit vdefs). Once you have the right one, just pick the subfolder with the latest definition date & revision and nab the full.zip file