Then I would do something like this

$vCenterList = Get-Content ".\vCenterList.txt"

foreach ($vCenter in $vCenterList){
    if($global:DefaultVIServers.Name -contains $vCenter){
        # Return existing connection
        $global:DefaultVIServers | where{$_.Name -eq $vCenter}
    }
    else
    {
        # Finally, connect to the VMware vCenter
        Connect-VIServer -Server $vCenterList
    }
}