Navigation with Xamarin Forms default MVVM

I recently started studying Xamarin Forms. I have a question that can be clarified here: how to navigate between pages using MVVM? Should your navigation be on vm or view ?

Example

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
     xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
     x:Class="GeoMobile.Views.Login">
     <StackLayout>
        <Label Text="GEO Mobile" VerticalOptions="Center" HorizontalOptions="Center" />
        <Entry Text="{Binding Email}" Placeholder="Email" />
        <Entry Text="{Binding Senha}" Placeholder="Senha" />
        <Button Text="Entrar" Command="{Binding Entrar}" />
        <Button Text="Registrar" />
     </StackLayout>
</ContentPage>

Login.cs

public partial class Login : ContentPage
{
    public Login()
    {
        InitializeComponent();
        this.BindingContext = new LoginVM(Entrar);
    }

    public async void Entrar()
    {
        Navigation.InsertPageBefore(new Master__(), this);
        await Navigation.PopAsync();
    }
}

And in my vm :

public class LoginVM : ObservableBase
{
    private string _email;

    public string Email
    {
        get { return _email; }
        protected set
        {
            _email = value;
            OnPropertyChanged();
        }
    }

    private string _senha;

    public string Senha
    {
        get { return _senha; }
        set
        {
            _senha = value;
            OnPropertyChanged();
        }
    }

    public Command Entrar { get; private set; }
    private Action loginOk;

    private void login()
    {
        //checagem antes de ir para a proxima pagina
        loginOk();
    }

    public LoginVM(Action loginOk)
    {
        this.loginOk = loginOk;
        Entrar = new Command(login);
    }
}

I got the navigation by passing an action in the constructor of vm, but I need to implement another navigation for the register button, but for that I would already have to pass another action in the constructor and I believe this would not be a good practice.

If you put the navigation in vm, the testing software process will be difficult. What is the best way to get around the problem?