Basically shared folders are renamed to synced folder from v1 to v2 (docs), under the bonnet it is still using vboxsf between host and guest (there is known performance issues if there are large numbers of files/directories).
/vagrant in guestVagrant is mounting the current working directory (where Vagrantfile resides) as /vagrant in the guest, this is the default behaviour.
See docs
NOTE: By default, Vagrant will share your project directory (the directory with the Vagrantfile) to /vagrant.
You can disable this behaviour by adding cfg.vm.synced_folder ".", "/vagrant", disabled: true in your Vagrantfile.
Based on the output /tmp on host was NOT mounted during up time.
Use VAGRANT_INFO=debug vagrant up or VAGRANT_INFO=debug vagrant reload to start the VM for more output regarding why the synced folder is not mounted. Could be a permission issue (mode bits of /tmp on host should be drwxrwxrwt).
I did a test quick test using the following and it worked (I used opscode bento raring vagrant base box)
config.vm.synced_folder "/tmp", "/tmp/src"
output
$ vagrant reload
[default] Attempting graceful shutdown of VM...
[default] Setting the name of the VM...
[default] Clearing any previously set forwarded ports...
[default] Creating shared folders metadata...
[default] Clearing any previously set network interfaces...
[default] Available bridged network interfaces:
1) eth0
2) vmnet8
3) lxcbr0
4) vmnet1
What interface should the network bridge to? 1
[default] Preparing network interfaces based on configuration...
[default] Forwarding ports...
[default] -- 22 => 2222 (adapter 1)
[default] Running 'pre-boot' VM customizations...
[default] Booting VM...
[default] Waiting for VM to boot. This can take a few minutes.
[default] VM booted and ready for use!
[default] Configuring and enabling network interfaces...
[default] Mounting shared folders...
[default] -- /vagrant
[default] -- /tmp/src
Within the VM, you can see the mount info /tmp/src on /tmp/src type vboxsf (uid=900,gid=900,rw).