Have you tried it this way?
$tab = "C:\Tab Server"
$tab = "D:\TabInstaller\TabServer-64bit.exe"
$tablog = "C:\Temp\Tableaulog\TabInstaller.log"
$list = "-noprofile -command ""Start-Process cmd.exe -Verb RunAs -ArgumentList '/c ""$tab"" /silent /install ACCEPTEULA=1 ACTIVATIONSERVICE=0 """"""INSTALLDIR=$($drive)"""""" ""/log $tablog""'"""
$proc = Start-Process "powershell.exe" -Credential $credential -ArgumentList $list -NoNewWindow -Wait -Passthru
if ($proc.ExitCode . . . ){
# do something
}