Hi,
We have found that changes Windows have made to their IPSEC/L2TP client in Vista introduce the same problems with the Mikrotik as encountered by Cisco. The following link describes the issue:
http://support.microsoft.com/kb/942429/en-us
and 3/4 of the way down the next link, Cisco refer to changes which had to be made to their IOS as a result:
http://forgenet.wordpress.com/category/cisco/page/4/
Two problems are observed; the first is a Vista machine can successfully establish an IPSEC/L2TP session with a Mikrotik, but will not be able to reconnect until the Installed SAs are flushed. The second is only one Vista machine can connect at the same time. Both these issues are related to the topic of the first link.
(Incidentally, a side issue is any changes to the peer settings causes (already connected) Remote Peers to disappear from the table, though they are still connected, and still appear as a dynamic l2tp interface)
We would respectfully request that Mikrotik address this issue in conjunction with the NAT-T problem.
Regards,
CAnder1