options_from_collection_for_select has 4 parameters:
- collection
- id
- column
- selected
You can provide a single value, or a hash to denote selected values. Try this:
<%= select_tag "vacancy_ids[]", options_from_collection_for_select(Vacancy.all,"id","title",{:selected=>[1,2,3,4]})), :multiple => true, :id => "vacancy_ids", :class => "form-control" %>
I'm not sure where the values you are trying to select come from but pipe them into the selected hash.