The mod makes Eldoth and Skie available the moment you leave Candlekeep. It moves Eldoth to the Friendly Arm Inn and expands his plan to kidnap / free Skie Silvershield. Follow his plan, and you will be able to get an alternative for Imoen and Safana sooner than in the original game. To make this happen, his quest had to be expanded, but it's easy enough to make low level party handle everything without any problems.
To keep the balance, Eldoth and Skie both start with level 1, however, to sweeten your game spiced with Eldoth's vile plans and Skie's bitter whining, she received a personal armor which makes it possible for her to help the team a bit more.
The mod was created mostly for personal use thus I mostly made sure the mod works well with mods in my current run. At that time I did not use BG1 NPC Project, and while they should work together, dialogues for Eldoth and Skie in BG1 NPC Project were written by other author(s), thus character interpretation and style may be different in those mods, however nothing controversial is included in the Vanishing.
I would also advise not to install any mod/component that changes Elsoth's starting location. Such components may cause problems, as The Vanishing also changes this part of the game.
That mod does not need Siege of Dragonspear to work properly.