In a good pot with a heavy base over medium heat, cook the apricot down with a little water just a few minutes for it to soften. Any lumps here will be lumps in the finished jam so if you don't like them, give them a mash!
Add the sugar, lemon juice and vanilla bean (scrapings and whole pod) and allow to bubble gently over low heat, stirring often to allow the fruit to jelly up. Do be careful here, as sugar can burn very easily and ruin a whole pot. Don't go too far from it and keep adjusting the temperature if need be.
Put a small saucer in the freezer.
About 10 minutes before your jam is ready, wash your jars in hot soapy water, rinse, and dry thoroughly. If you would like to make the jam shelf-stable, pop the jars and lids in a hot oven for about 10 minutes.
After about 20 minutes, put a little teaspoon of jam on your frozen saucer, if you can drag your finger through it and it stays separated, you're good to go. Otherwise keep bubbling away and checking on a cold saucer every 10 minutes to get the consistency you want. It will thicken up even more when cool so keep that in mind.
When your jam is thick enough, remove your vanilla bean and ladle the jam carefully into your piping hot jars and put the lids on tight. Leave them for 24 hours untouched to form a seal. Usually you can tell when the raised part of the jar lid has been sucked down (you can also hear the pop when it happens - always a good sound!). If you're unsure or the lid hasn't sealed properly after they've cooled, store in the refrigerator.