Final drive 2003 vstar custom losted all final drive fliud

Http://www.sloneservices.com/SilverBack/Other-Stuff/V-StarShopManual99-07.pdf

The link will take you to a service manual, I suggest downloading it. Good info in it.
First, If the oil came out of the breather, you need to find out if it was due to a faulty breather or through natural loss (you will lose gear oil over time through rapid temperature increase causing vapor and pressure increase-eventually oil loss will occur through the breather; synthetic oil can help reduce this, but so can regular level checks. If the breather is defective, replace with a new one (10-20 bucks depending on where you go) it is a press fit.
Second, if it came out of the fill plug on the back of the gear case, check the threads on the gear case as well as the plug, if both are good, check the condition of the collar (brass/copper looking ring on the plug. If it is missing, that could contribute to the oil loss. When you reassemble, use teflon tape to ensure a good tight fit. (mine has teflon tape on there too-only use enough to coat the threads...if you use too much, the plug will not seat all the way due to the tape blocking the threads)
Third, do the same check on the drain plug on the bottom. Reassemble with teflon tape on the plug.
Finally, on page 272 of that same manual in the link, there are procedures to follow for checking the drive shaft seal for leaks by the case. Check it and replace seals if required.