So the standard 512Mb of memory are not enough for you ? You want to allocate more memory for your Vagrant created VM ?
Just add this inside Vagrant::Config.run do |config| on your Vagrantfile:
config.vm.customize ["modifyvm", :id, "--memory", 1024]
In this example we are assigning 1Gb of total available memory (You can change the number to suit your needs ;)