Did you check that the DefaultVIServerMode is set to Multiple?
And with $global:defaultviservers you can check to which vCenters you are connected.
$vCenters = 'vc1','vc2'
Set-PowerCLIConfiguration -DefaultVIServerMode Multiple -Confirm:$false
Connect-VIServer -Server $vCenters -User xxxxx -Password xxxxx
$global:DefaultVIServers