Don't think lastpass is integrated within mobile IE like you can with dolphin browser on android. Also, you'd have to pay for the premium lastpass to get the mobile version.
You can open a site within the last pass app but I can't figure out how to get that to go to the full 'open in IE' mode.
In IE, maybe you have selected 'dont save cookies' or maybe have to unselect 'do not track' for that site to pre-populate log in data.
Don't want it to sound like I'm bashing Laspass though. It has made my digital life so much easier and more secure that I've been a paying premium subscriber for a long time. Just not sure it's the answer for this. Unless it's better integrated on WP8 IE than it seems like from my experience....