Hi Claudio
This question vlan trunking yes or no was indeed weird; the internal implementation of UCS uses always trunking, therefore this question doesn't make sense, and was removed.
If your OS can handle a vlan trunk on a vnic, just select all the vlans you need.
If you have a OS with lots of vnics, with only one vlan per vnic, you tag them as native (therefore no tagging). Your OS is not configured for vlan trunking.
Cheers Walter.