PPTP network for server backend LAN?

Here is our problem:

We have several webservers, which should be reached from public. The database servers that store the data for the web apps on those webservers though shall not have a public IP.

So, since I want to be able to connect to the SQL servers using ssh for example, and those servers need to talk with each other, I had this idea:

Internet
|
------------------
|                |
Webserver 1      Webserver 2     Database Server
|                |               |
-------------- vLAN --------------
|
PPTP
|
Workstation (my PC)

My idea was that I can connect to the vLAN using PPTP so that I have access to all servers in that LAN, but the database server remains unvisible to the public.

Is this infrastructure a good idea?

You just described a DMZ. There's no need to vpn to get to this. Simple routing from the internal network to the DMZ is normally sufficient.

A typical company network looks like this.

Internet -- firewall -- dmz
               |
      protected networks

The only time I would resort to a VPN is to access the protected networks or management services/ports on the dmz servers from somewhere on the internet....like from home.