Try the following VCL config. This is for Vanish 4 and I haven't tested this yet, so make a backup of your existing VCL file.

import directors;

backend web1 {
    .host = "1.1.1.1";
    .port = "80";
}

backend web2 {
    .host = "2.2.2.2";
    .port = "80";
}

sub vcl_init {
    new apache = directors.random();
    apache.add_backend(web1, 3);
    apache.add_backend(web2, 1);
}