Did find out: gui of the switch 2401, delete members of PortChannel -> apply -> go to 2nd lag/lacp add members, reconfigure cisco port channel et voilà !  how ever still asking myself why it does not show only 4 vmnic/blade, I guess internal ports of the blades...???
Hi Everyone, we have currently 2 blades inside our vrtx (m630 and m520), we had 4 ports connected -Internal 2401 Switch- to Cisco 2960x through a portchannel (trunk) and they work very well. We decided to give a try and 4 more with a second portchannel : did succeed on either side (cisco and dell 2401 - all up and no errors ) but: We are running initial fw of 2401 the lag / lacp gives only capacity of assigning max 4 ports per lag (other ports you have to select lag 2) We find a way / a procedure to understand how esxi 6.5 can discover those 4 additionals vmnics (did cli : not listed at all, did reboot one of the blade which is not used today : no discovery even with lspci command, only the  4 initial ports). full cold restart of the vrtx ??? any guess ?   
Vrtx and adding 4 additional connections through r1-2401 (esxi 6.5)