Matt
I do not know of an easy way to accomplish this. The solution I will suggest might work depending on what you are logging on your ASA and on appropriate access to the log records. I have done something like this as a manual effort for a customer and perhaps you could create something that would automate the process.
What I did was to access log records that the ASA sent to a log server (could not get the appropriate time span on the ASA) and would search for a particular log message number that would be present only once for a user's VPN session (perhaps a log record generated when the user authenticated or perhaps a log message about assignment of an IP address to the user). I would write the log records to a file where I could manipulate them. You could perhaps extract the user name and sort on that or do something to eliminate duplicate entries where a user had multiple VPN sessions on that day.
HTH
Rick
HTH
Rick