Attending a scientific conference is an activity permitted under the Visa Waiver Program, which you as a German citizen are entitled to use. This includes presenting at a conference, assuming you are not being paid directly for doing so. Tourism is also a permitted activity so you can do either or both on any visit.
If the combined total of your stays in the US and your visits to the US and British Virgin islands are less than 90 days then you can use the VWP for the entire trip.
You need to apply for an ESTA. Then you fly to the US, are admitted there, and a record of your VWP admission is created. When you return to US territory after your visit the Caribbean they will find the record of your previous admission and readmit you. You would need to leave the US within 90 days of your previous admission.
Things would be different if you were driving into the US initially. There are some rare cases where VWP nationals are unable to use the VWP, such as visiting a banned country or having a criminal record. You will discover this when you apply for the ESTA if it applies to you, so do it early if there is any chance it might.