If you choose to become a vampire before entering the Soul Cairn while already allied with the Dawnguard, there are several consequences that you will face. First and foremost, you will be kicked out of the Dawnguard faction. The Dawnguard is an organization dedicated to hunting and eradicating vampires, so it is natural that they would not tolerate one of their own becoming a vampire. This decision will have a significant impact on your gameplay and relationships with the Dawnguard members.
Once you become a vampire, the Dawnguard members will no longer trust you and will view you as an enemy. They will attack you on sight, making it difficult for you to navigate their headquarters or interact with any of the members. This means you will no longer have access to their resources, such as training, quests, and unique items. It also means that any potential companions within the Dawnguard will no longer be available to you.
Additionally, your relationship with Serana, the vampire who is attempting to smuggle you into the Soul Cairn, will change. Serana is a unique character who can be a follower and potential love interest. However, if you choose to become a vampire, she will no longer be your follower, as she is aligned with the Dawnguard and will also turn hostile towards you. This can be a significant loss, as Serana is a powerful and interesting character with her own questline.
Being a vampire also comes with its own set of advantages and disadvantages. As a vampire, you will gain various supernatural powers and abilities, such as increased strength, speed, and the ability to drain life from your enemies. However, you will also face significant drawbacks. Vampirism makes you weaker to fire and sunlight, and you will need to regularly feed on the blood of the living to maintain your powers and prevent yourself from becoming too weak. This can be challenging, as feeding on innocent NPCs can have negative consequences and make you more feared and hated in the world of Skyrim.
If you choose to become a vampire before entering the Soul Cairn, you will still be able to complete the main questline of the Dawnguard DLC. However, it will be a more challenging experience, as you will need to navigate the hostility of the Dawnguard members and find alternative ways to progress through the quests. It is worth noting that if you wish to continue working with the Dawnguard, you will eventually need to cure yourself of vampirism.
Choosing to become a vampire before entering the Soul Cairn while already allied with the Dawnguard will result in being kicked out of the faction, losing access to their resources and members, and facing hostility from both the Dawnguard and Serana. It will also present challenges and drawbacks associated with being a vampire. Ultimately, it is up to you to decide whether the benefits of vampirism outweigh the consequences and if you are willing to navigate the difficulties that come with it.