From everything I've seen, vbscript isn't supported with the move to PowerShell. You could use vbs to call PowerShell and run the appropriate cmdlet if you really need to use vbscript. Other than that you'll probably want to look at a different solution.
$password = Read-Host "Enter password" -AsSecureString
New-Mailbox -UserPrincipalName testuser@mlexchange.net -Alias testuser -Database "Mailbox Database 2048259302" -Name testuser –OrganizationalUnit Users -Password $password -FirstName test -LastName user -DisplayName "Test User" -ResetPasswordOnNextLogon $true