use the button in the LastPass notification bar that only fills in the login details, not the one that fills and then submits
Is there no way to get LastPass to automatically fill in that data?
Oh, I see. Sorry. I'd misunderstood. So whenever you visit the web page or select it from the "Sites" menu on the LastPass menu then you want LastPass to automatically fill in the user_ID field but to stop there, i.e. leave the password field blank for you to fill in and of course you don't want it to try and press any buttons to do a submit of the login details because that has to be left to you to do once you've typed in the password.
I'm afraid I don't know the answer to that one so we'll have to hope that Joe or someone else from LastPass posts a reply. I would say (to LastPass) though that, if they did implement a "Never AutoSubmit" flag for the vault login records (the aforementioned JSL01) then the current mechanisms that do auto-login would instantly start doing exactly what you want if you also set the (currently non-existent) "Never AutoSubmit" flag for the sites where you want this behaviour.
One final point regarding your concern about storing the passwords themselves in LastPass. It isn't entirely a one-way bet in terms of reducing the risk of your passwords being compromised, it's a balancing act.
Despite all the assurances, it's always going to be true that not divulging a piece of sensitive information to any third party, however much you trust that organisation, will increase the privacy (security) of that information however marginally (and in LastPass's case that is only very, very marginally because the only thing you divulge to them is heavily encrypted data). However, don't forget to balance that infinitesimally small gain against the ways in which keeping your passwords in LastPass increases your security, possibly to an extent that outweighs the reduction due to putting them in LastPass. In particular, the ways that you might be compromising your security by not using Lastpass are: (1) You make yourself more vulnerable to key loggers that might infect the machine you are using, and (2) Lastpass makes it easier to use stronger passwords (because you don't have to remember them).
Just my two cents worth.