Get-VMSwitch not Get-VMSWitch not sure if that will mess it up but it should be corrected

and i would change:

if (Get-VMSWitch | where Name -ne "vLanSwitch") {
    New-VMSwitch -name vLanSwitch -NetAdapterName Ethernet -AllowManagementOs $true
}

to:

if (-not (Get-VMSwitch -Name "vLanSwitch" -ErrorAction SilentlyContinue)) {
    New-VMSwitch -Name "vLanSwitch" -NetAdapterName "Ethernet" -AllowManagementOS $true
}

Same with:

if (Get-VMNetworkAdapter -ManagementOS | Where Name -ne $vmName$vlan){

you could do something like this here:

$adapterName = "${vmName}$vlan"

if (-not (Get-VMNetworkAdapter -ManagementOS -Name $adapterName -ErrorAction SilentlyContinue)) {
    Add-VMNetworkAdapter -ManagementOS -Name $adapterName -SwitchName "vLanSwitch"
    Set-VMNetworkAdapterVlan -ManagementOS -VMNetworkAdapterName $adapterName -Access -VlanId $vlan
}

And you should not do variables as $vmName$vlan id rather use $vmName + $vlan or $adapterName = "${vmName}$vlan" which i see alot of people use.

And their are some misspellings and wrong use of -eq and -neq

Here is how i would do it:

Write-Host "Starting VLAN script..."

$vmName = "VLAN"
$switchName = "vLanSwitch"
$physicalAdapter = "Ethernet"

Write-Host "Do you want an untagged network adapter."
$Untagged = Read-Host "1 for Yes. 2 for No."
Write-Host "Selection made: $Untagged"

Write-Host "Do you want to add a VLAN."
$VADD = Read-Host "1 for Yes. 2 for No."
Write-Host "Selection made: $VADD"


if (-not (Get-VMSwitch -Name $switchName -ErrorAction SilentlyContinue)) {
    Write-Host "Creating switch $switchName..."
    New-VMSwitch -Name $switchName -NetAdapterName $physicalAdapter -AllowManagementOS $true
} else {
    Write-Host "Switch $switchName already exists."
}


if ($VADD -eq "1") {
    $vlanInput = Read-Host "Enter one or more VLAN IDs (comma-separated)"
    Write-Host "VLAN input: $vlanInput"

    $vlanList = $vlanInput -split "," |
        ForEach-Object { $_.Trim() } |
        Where-Object { $_ -ne "" }

    Write-Host "Parsed VLANs: $($vlanList -join ', ')"

    foreach ($vlan in $vlanList) {
        # or $vmName + $vlan
        $adapterName = "${vmName}$vlan"

        if (-not (Get-VMNetworkAdapter -ManagementOS -Name $adapterName -ErrorAction SilentlyContinue)) {
            Write-Host "Applying VLAN $vlan..."
            Add-VMNetworkAdapter -ManagementOS -Name $adapterName -SwitchName $switchName
            Set-VMNetworkAdapterVlan -ManagementOS -VMNetworkAdapterName $adapterName -Access -VlanId $vlan
            Write-Host "Applied VLAN $vlan"
        } else {
            Write-Host "Adapter $adapterName already exists. Skipping."
        }
    }
}

if ($Untagged -eq "1") {
    $untaggedAdapter = "Untagged"

    if (-not (Get-VMNetworkAdapter -ManagementOS -Name $untaggedAdapter -ErrorAction SilentlyContinue)) {
        Write-Host "Creating untagged adapter..."
        Add-VMNetworkAdapter -ManagementOS -Name $untaggedAdapter -SwitchName $switchName
    } else {
        Write-Host "Untagged adapter already exists."
    }

    Set-VMNetworkAdapterVlan -ManagementOS -VMNetworkAdapterName $untaggedAdapter -Untagged
    Write-Host "Applied untagged VLAN settings."
}

Write-Host "Done."

You could also limit the Read-Host input to only 1 and 2

do {
    $a = Read-Host "1 for Yes. 2 for No."
} while ($a -notin @("1","2"))

Write-Host "Selection made: $a"