The Events Calendar: All Venue slugs get prefixed with a custom string. Works in wp-admin and Community Events.

functions.php

<?php
/**
 * The Events Calendar: All Venue slugs get prefixed with a custom string. Works in wp-admin and Community Events.
 *
 * !!! Change the prefix to your own !!!
 *
 * @link https://gist.github.com/cliffordp/ac4a68769787352369ab0c5e41dc28ab This snippet.
 */
function cliff_prefix_venue_slugs( $data ) {
	// TODO: Change this to what you want!!!
	$prefix = 'xvenue-';

	if (
		(
			'publish' === $data['post_status']
			|| 'draft' === $data['post_status']
			|| 'pending' === $data['post_status']
		)
		&& class_exists( 'Tribe__Events__Venue' )
		&& Tribe__Events__Venue::POSTTYPE === $data['post_type']
		&& 0 !== strpos( $data['post_name'], $prefix ) // is not already prefixed, such as when moving from Deleted back to Published
	) {
		$data['post_name'] = $prefix . $data['post_name'];
	}

	return $data;
}

add_filter( 'wp_insert_post_data', 'cliff_prefix_venue_slugs' );