How to increase your heartbeat timeout for your ESX hosts to vCenter. Normally every ESX host sends a heartbeat every 10 sec and the virtual center (vCenter) waits 20sec for that heartbeat. This should not be a problem if you have a good network or good links for remote sites. But if you have a slow link to a remote site that can mean that your hosts might show up as not responding when they are.
The fix:
Note! do a back up of your vpxd.cfg file first
locate the vpxd.cfg file at C:\Documents and Settings\All Users\Application Data\VMware\VMware VirtualCenter\ and add in the <vpxd> </vpxd>
<heartbeat>
<notRespondingTimeout>60</notRespondingTimeout>
</heartbeat>
Then restart the virtualcenter server service
[...] into a not responding state in vCenter Server due to WAN bandwidth or latency issues. Read more here. Example: 60 [...]