How can I make a TextBox be a "password box" and display stars when using MVVM?

To get or set the Password in a PasswordBox, use the Password property. Such as

string password = PasswordBox.Password;

This doesn't support Databinding as far as I know, so you'd have to set the value in the codebehind, and update it accordingly.

Send the passwordbox control as a parameter to your login command.

<Button Command="{Binding LoginCommand}" CommandParameter="{Binding ElementName=PasswordBox}"...>

Then you can call CType(parameter, PasswordBox).Password in your viewmodel.

There's a way to bind on a PasswordBox here: PasswordBox Databinding

As Tasnim Fabiha mentioned, it is possible to change font for TextBox in order to show only dots/asterisks. But I wasn't able to find his font...so I give you my working example:

<TextBox Text="{Binding Password}" 
     FontFamily="pack://application:,,,/Resources/#password" />

Just copy-paste won't work. Firstly you have to download mentioned font "password.ttf" link: https://github.com/davidagraf/passwd/blob/master/public/ttf/password.ttf Then copy that to your project Resources folder (Project->Properties->Resources->Add resource->Add existing file). Then set it's Build Action to: Resource.

After this you will see just dots, but you can still copy text from that, so it is needed to disable CTRL+C shortcut like this:

<TextBox Text="{Binding Password}" 
     FontFamily="pack://application:,,,/Resources/#password" > 
    <TextBox.InputBindings>
        <!--Disable CTRL+C (COPY) -->
        <KeyBinding Command="ApplicationCommands.NotACommand"
            Key="C"
            Modifiers="Control" />
        <!--Disable CTRL+X (CUT) -->
        <KeyBinding Command="ApplicationCommands.NotACommand"
            Key="X"
            Modifiers="Control" />
    </TextBox.InputBindings>
    <TextBox.ContextMenu>
        <!--Hide context menu where you could copy/cut as well -->
        <ContextMenu Visibility="Collapsed" />
    </TextBox.ContextMenu>
</TextBox>

EDIT: Added Ctrl+X and Context menu disabling based on @CodingNinja comment, thank you.