Out of memory when vacuuming

Hi,

I upgraded my server from 7.4.7 to 8.1.4 and when I want to run a vacuum
analyze verbose I've got this error message:
INFO:  vacuuming "public.lgevents"
ERROR:  out of memory
DETAIL:  Failed on request of size 262143996.


My settings:
shared_buffers = 300000
temp_buffers = 256000
work_mem = 256000
maintenance_work_mem = 256000
max_fsm_pages =  250000
effective_cache_size = 614400

My server  has 16Gbyte memory, and kernel.shmmax = 4294967295
Do you have any idea why I got this error message

Thanks,
Szabek