Multi VM in one Vagrantfile. Could I set different memory size for each of them?

2016-08-31: Updated answer to include the whole Vagrantfile per @DarkForce request.

You can do that by moving the vm.provider definition inside each of the vm.define blocks. For example this configuration sets the memory to 2048MB for "web" and 1024MB for "db":

# -*- mode: ruby -*-
# vi: set ft=ruby :

Vagrant.configure(2) do |config|
  config.vm.box = "ubuntu/trusty64"

  config.vm.define :web do |web_config|
      web_config.vm.host_name = "web"
      web_config.vm.network "private_network", ip:"192.168.100.10"
      web_config.vm.provider :virtualbox do |vb|
          vb.customize ["modifyvm", :id, "--memory", "2048"]
          vb.customize ["modifyvm", :id, "--cpus", "2"]
      end
  end

  config.vm.define :db do |db_config|
      db_config.vm.host_name = "db"
      db_config.vm.network "private_network", ip:"192.168.100.20"
      db_config.vm.provider :virtualbox do |vb|
          vb.customize ["modifyvm", :id, "--memory", "1024"]
          vb.customize ["modifyvm", :id, "--cpus", "2"]
      end
  end
end

Note: This example (like many in the Vagrant docs) will only work for VirtualBox. If you wanted your Vagrantfile to also work with VMware or another provider, the customization parameters would be listed separately. For example:

x.vm.provider "vmware_fusion" do |v|
  v.vmx["memsize"] = "3000"
end
x.vm.provider :virtualbox do |v|
  v.customize ["modifyvm", :id, "--memory", "3000"]
end

The best way is to use Ruby array of hashes like here - http://sysadm.pp.ua/linux/sistemy-virtualizacii/vagrantfile.html . You can define array like:

    servers=[
      {
        :hostname => "web",
        :ip => "192.168.100.10",
        :box => "saucy",
        :ram => 1024,
        :cpu => 2
      },
      {
        :hostname => "db",
        :ip => "192.168.100.11",
        :box => "saucy",
        :ram => 2048,
        :cpu => 4
      }
    ]

Then you just iterate each item in server array and define configs:

Vagrant.configure(2) do |config|
    servers.each do |machine|
        config.vm.define machine[:hostname] do |node|
            node.vm.box = machine[:box]
            node.vm.hostname = machine[:hostname]
            node.vm.network "private_network", ip: machine[:ip]
            node.vm.provider "virtualbox" do |vb|
                vb.customize ["modifyvm", :id, "--memory", machine[:ram]]

Vagrant also has convenient shortcuts for memory and CPU settings that take the following form:

config.vm.provider "virtualbox" do |v|
  v.memory = 1024
  v.cpus = 2
end

So in your case, and to give twice the CPUs and memory to DB boxes than web boxes, it would look something like:

config.vm.define :web do |web_config|
    web_config.vm.box = "saucy"
    web_config.vm.host_name = "web"
    web_config.vm.network "private_network", ip:"192.168.100.10"
    web_config.vm.provider :virtualbox do |vb|
        vb.memory = 1024
        vb.cpus = 2
    end
end

config.vm.define :db do |db_config|
    db_config.vm.box = "saucy"
    db_config.vm.host_name = "db"
    db_config.vm.network "private_network", ip:"192.168.100.20"
    db_config.vm.provider :virtualbox do |vb|
        vb.memory = 2048
        vb.cpus = 4
    end
end

I just tried to use the above solutions and realized, that in my environment all memory settings are the same. The code snippets seem to have a small typo - do not use config.vm.provider ... but use <yourConfigVariable>.vm.provider... to set the machine dependent values.

AFAIK the correct code would look like this:

config.vm.define :web do |web_config|
    web_config.vm.box = "saucy"
    web_config.vm.host_name = "web"
    web_config.vm.network "private_network", ip:"192.168.100.10"
    web_config.vm.provider :virtualbox do |vb|
        vb.customize ["modifyvm", :id, "--memory", "2048"]
        vb.customize ["modifyvm", :id, "--cpus", "2"]
    end
end

config.vm.define :db do |db_config|
    db_config.vm.box = "saucy"
    db_config.vm.host_name = "db"
    db_config.vm.network "private_network", ip:"192.168.100.20"
    db_config.vm.provider :virtualbox do |vb|
        vb.customize ["modifyvm", :id, "--memory", "1024"]
        vb.customize ["modifyvm", :id, "--cpus", "2"]
    end
end