What happens next?

How to fish wires through a conduit or pipe
Unwind several feet of the light string and feed a little bit of it into one end of the pipe.
Start up the vacuum cleaner and attach it to the other end of the pipe.
Try to seal the connection the best you can to make the most efficient use of the vacuum.
The vacuum should pull the string through the pipe.. Watch the end where the string is feeding into the pipe to make sure it doesn't get snagged and stop.