Something like this might work for you:
$vpnName = "MyVPN";
[array]$vpns = Get-VpnConnection |
Where-Object {$_.Name -Like "*$vpnName" -AND $_.ConnectionStatus -eq "Connected"}
if ($vpns.count -eq 1){
rasdial $vpnName /DISCONNECT
}
else{
Write-Host "Multiple, or no, ($($vpns.count)) VPN connections match '$vpnName'"
$vpns.Name
}