foreach (var item in list1)
{
    if (list2.Where(c => c.vopros == item.vopros).Count() == 0)
    {
        list3.Add(item);
    }
}

foreach (var item in list1)
{
    if (list2.Exists(c => c.vopros == item.vopros))
    {
        continue;
    }

    list3.Add(item);
}

ЦИКЛ ТО
    ПОКА УСЛОВИЕ... 

public struct Doc
{
    public readonly string vopros;

    public Doc(string vopros)
    {
        this.vopros = vopros;
    }

    public override int GetHashCode()
    {
        return vopros.GetHashCode();
    }
}

var set = new HashSet<Doc>(Enumerable.Range(0, 2000000).Select(x => new Doc(x.ToString())));
var stopwatch2 = new Stopwatch();
stopwatch2.Start();

set.ExceptWith(list2);

stopwatch2.Stop();

Console.WriteLine($"With HashSet it takes {stopwatch2.Elapsed.TotalMilliseconds.ToString()} milliseconds");