Vacuum never completed....

We had a server approach wraparound yesterday on a 9.3 box. I restarted 
in single user mode and only one table was above 
autovacuum_freeze_max_age. The dataset was ~750G. We left vacuum running 
for 21hrs and it still did not complete. What was interesting is that 
disk IO graph kept cycling, lots of reads for ~45min, then a few writes 
for another 45min. I failed to VACUUM VERBOSE so there was little other 
data to go on other than IO graph. We ended dropping the table and 
lowering autovacuum_freeze_max_age for clean up. We've had similar sized 
dataset complete vacuum in a few hours, but I've never seen this cycling 
IO before: https://imagebin.ca/v/2eIdB2bjFBxq

Is this normal? It gave me very little faith that VACUUM would ever 
complete.