Can I modify the ownership for a shared folder in vagrant?

As @StephenKing suggests you can change the options of the whole directory.

The relevant function is not documented but the source tells us:

# File 'lib/vagrant/config/vm.rb', line 53

def share_folder(name, guestpath, hostpath, opts=nil)
  @shared_folders[name] = {
    :guestpath => guestpath.to_s,
    :hostpath => hostpath.to_s,
    :create => false,
    :owner => nil,
    :group => nil,
    :nfs   => false,
    :transient => false,
    :extra => nil
  }.merge(opts || {})
end 

Basically you can set group, owner and acl for the whole folder which is way better than setting everything to world writable on the host. I have not found any method to change the ownership of a nested directory.

Here is a quickfix:

config.vm.share_folder "v-wordpress", "/var/www/wordpress", "/host/path", :owner => "www-data", :group => "www-data"