<?php
//Retrieve venues that match query criteria
$args = array(
  'nopaging' => true,
  'post_type'=>'tribe_venue',
  //Only query venues in Frankfurt
  'meta_query' => array(
    array(
       'key' => '_VenueCity',
       'value' => array( 'Frankfurt'),
       'compare' => 'IN',
    )
  )
);
$city_venue = get_posts( $args );
$venue_ids = wp_list_pluck( $city_venue, 'ID' );

wp_reset_postdata();

$upcoming = new WP_Query( array(
  'post_type' => Tribe__Events__Main::POSTTYPE,
  'meta_query' => array(
    array(
       'key' => '_EventVenueID',
       'value' => $venue_ids,
       'compare' => 'IN',
    )
  ),
  'paged' => $paged
) );