ho comunque trovato la "causa": nel vmkernel.log c'erano, da ore prima del disastro sulla vmnic4, questi messaggi ripetuti che posto più in basso.
Sembra sia un problema diffuso fra le broadcom 5719/5720, il driver tg3 e gli switch dell.
Il workaround pare quello di disabilitare il NetQueue sull'host...
ESXi 5 vmnic stops passing traffic - HP DL380p Gen8 - HCL fine
2013-07-17T07:54:42.115Z cpu14:8240)<6>tg3 : vmnic4: NetQ remove RX filter: 4
2013-07-17T07:54:42.115Z cpu14:8240)<6>tg3 : vmnic4: Free NetQ RX Queue: 4
2013-07-17T07:55:17.114Z cpu12:8240)<6>tg3 : vmnic4: NetQ remove RX filter: 3
2013-07-17T07:55:17.114Z cpu12:8240)<6>tg3 : vmnic4: Free NetQ RX Queue: 3
2013-07-17T07:55:37.112Z cpu8:8240)<6>tg3 : vmnic4: RX NetQ allocated on 3
2013-07-17T07:55:37.113Z cpu8:8240)<6>tg3 : vmnic4: NetQ set RX Filter: 3 [00:50:56:6a:45:8b 0]
2013-07-17T07:55:47.113Z cpu4:8240)<6>tg3 : vmnic4: NetQ remove RX filter: 3
2013-07-17T07:55:47.113Z cpu4:8240)<6>tg3 : vmnic4: Free NetQ RX Queue: 3
2013-07-17T07:56:12.111Z cpu4:8240)<6>tg3 : vmnic4: RX NetQ allocated on 3
2013-07-17T07:56:12.111Z cpu4:8240)<6>tg3 : vmnic4: NetQ set RX Filter: 3 [00:50:56:6a:45:8b 0]
2013-07-17T07:56:27.112Z cpu9:8240)<6>tg3 : vmnic4: NetQ remove RX filter: 3
2013-07-17T07:56:27.112Z cpu9:8240)<6>tg3 : vmnic4: Free NetQ RX Queue: 3
2013-07-17T07:57:52.106Z cpu10:8240)<6>tg3 : vmnic4: RX NetQ allocated on 3
2013-07-17T07:57:52.107Z cpu10:8240)<6>tg3 : vmnic4: NetQ set RX Filter: 3 [00:50:56:6a:45:8b 0]
2013-07-17T07:57:57.107Z cpu4:8240)<6>tg3 : vmnic4: NetQ remove RX filter: 3
2013-07-17T07:57:57.107Z cpu4:8240)<6>tg3 : vmnic4: Free NetQ RX Queue: 3
2013-07-17T07:58:02.106Z cpu1:8240)<6>tg3 : vmnic4: RX NetQ allocated on 3
2013-07-17T07:58:02.107Z cpu1:8240)<6>tg3 : vmnic4: NetQ set RX Filter: 3 [00:50:56:90:00:31 0]
2013-07-17T07:58:07.106Z cpu1:8240)<6>tg3 : vmnic4: NetQ remove RX filter: 3
2013-07-17T07:58:07.106Z cpu1:8240)<6>tg3 : vmnic4: Free NetQ RX Queue: 3
2013-07-17T08:00:02.102Z cpu11:8240)<6>tg3 : vmnic4: RX NetQ allocated on 3
2013-07-17T08:00:02.102Z cpu11:8240)<6>tg3 : vmnic4: NetQ set RX Filter: 3 [00:50:56:6a:45:8b 0]