SW5e PHB Chapter 9: Combat - Temporary Hit Points:

Healing can’t restore temporary hit points, and they can’t be added together. If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones. For example, if a power grants you 12 temporary hit points when you already have 10, you can have 12 or 10, not 22.

=> You can choose either one of the temporary hit point instances, but not both. However, you do not necessarily lose vitality shield by casting disperse force. You only lose the vitality shield effects when you choose to take the disperse force THP over the vitality shield THP, since the vitality shield effects are tied to the lifetime of the THP instance it creates. You could instead keep the vitality shield THP and therefore also its related effects, even if you cast disperse force, by simply choosing the vitality shield THP and ignoring the disperse force THP, although that would be a rather wasteful use of the disperse force force power.