The error was XCache wasn't configured with a var size. You'll need to edit your php.ini, look for xcache.var_size, and give it a size, something like 32 would be fine:
Once done, restart apache. Then vBO should be able to use XCache just fine
hmm this is my php.ini
# configuration for php Xcache module
extension = xcache.so
[xcache.admin]
xcache.admin.auth = On
# Configure this to use admin pages
; xcache.admin.user = "mOo"
; xcache.admin.pass = md5($your_password)
; xcache.admin.pass = ""
[xcache]
; ini only settings, all the values here is default unless explained
; select low level shm/allocator scheme implemenation
xcache.shm_scheme = "mmap"
; to disable: xcache.size=0
; to enable : xcache.size=64M etc (any size > 0) and your system mmap allows
xcache.size = 16M
; set to cpu count (cat /proc/cpuinfo |grep -c processor)
xcache.count = 1
; just a hash hints, you can always store count(items) > slots
xcache.slots = 8K
; ttl of the cache item, 0=forever
xcache.ttl = 0
; interval of gc scanning expired items, 0=no scan, other values is in seconds
xcache.gc_interval = 0
; same as aboves but for variable cache
xcache.var_size = 32M
xcache.var_count = 1
xcache.var_slots = 8K
; default ttl
xcache.var_ttl = 0
xcache.var_maxttl = 0
xcache.var_gc_interval = 300
check the phpinfo() :
http://forum.quadmania.com/info.php