I have a vendor in my vault. When I click "launch", the vendor's log-in page comes up with the LastPass popup window, all normally, but at the bottom of the window it states "There's nothing in your vault to fill on this site". However, the vendor is in my vault with all of the correct information.
Could you try the steps outlined in this support article to see if it resolves the issue with Autofill not working on certain sites? Please note that not all sites support LastPass' autofill due to how they are coded/designed: https://support.logmeininc.com/lastpass/help/lastpass-wont-save-or-autofill-my-data-for-a-site-lp07...
So far nothing there has worked. Lastpass fails to autofill on some HTTPS sites and not others. Have web designers have found a new way to block Lastpass? I've spent a lot of time trying to find a setting that could be causing the problem . Do Lastpass devs ever see this problem? If the answer is no, please tell us and we can look at addons or other personal differences...
Sometimes sites start using a different name or type of field to ask you for your credentials, which LP does not track. Although I think LP should pick up on ALL fields, it's not always easy to distinguish from a regular form fill or actual credentials input field.
I had it with a vendor, that used to work for a long time, but in the mean time the email login field has changed identity a few times.
You might throw away completely the site in your LP configuration, and have it recognize as new entry. But that does not always work I found out.
A way around is to see what name is used for the field and add it to your password entry in LP.
Bit technical, but look it up in the page source, or rather use F12/Developer mode 
Use the picker  (pick an element in FF, first icon) and point it at the field to be filled in 
See what the input field is named and record that. You see something like in my example <input id="email_verify" class="inputfield" .. but that may very in your case. Important to look at if there is something like an 'id'.
Next you edit the password entry of the site that should have been filled in.Select the wrench 'Edit Form Fields' option at the bottom.
Add a Form Field entry (with preferred value) if the field from step 1 is not listed. Check the value if it does: