I use the VPN to access my company network and works good, I tried manually going to sleep and then awakening back and the connection is still there, then letting go auto-lock and worked good.
I'm using a profile provided by company IT.
Now, we use a tunnel for security reasons and of course if your current IP changes for any reason (going cellular to Wi-Fi or viceversa) of course the tunnel goes down and you need to re-open the VPN.
We also use a soft token as password for bringing the tunnel up so it is a little bit cumbersome but it is the same as using accessing the VPN with a laptop, either in Windows or OSX. I guess if we were to use a certificate the VPN should connect immediately but I have no way to tell.
So I think the problem may be related on the characteristic of the VPN. Just an opninion.