// Get Brewer Phone Numbers
String brewfestName = 'Brewfest 2017';
List<Brew__c> brewers = [select Id, Name, Brewer_Phone__c
                        from Brew__c
                        where Brewfest__r.Name = :brewfestName
                        and Brewer_Phone__c != null];
System.debug( 'Number of brewers is ' + brewers.size());

// Get the unique phone numbers
Set<String> brewerPhones = new Set<String>();
for ( Brew__c brew : brewers) {
    brewerPhones.add( brew.Brewer_Phone__c);
}
System.debug( 'Number of unique brewer phones is ' + brewerPhones.size());

// Get Votes by the Brewers
List<Vote__c> brewerVotes = [select Id
                             from Vote__c
                             where Brew__r.Brewfest__r.Name = :brewfestName
                            and Voting_Phone__c in :brewerPhones];
System.debug( 'Number of brewer votes is ' + brewerVotes.size());

// Set the Brewer Vote field on Vote
for ( Vote__c brewVote : brewerVotes) {
    brewVote.Brewer_Vote__c = true;
}
update brewerVotes;