The speaker is attempting to convey the impact of the protagonists personality on her own psyche:
2 A marked effect or influence:
The impact is expressed metaphorically in terms of the effect that vindaloo curry has on the speaker's palate: pleasant in small doses, but painful in large doses. Apparently the power of the protagonist's personality "burns" the speaker's psyche in a metaphorical sense, so that the she avoids interaction for short seasons, but not so much that she abandons the friendship altogether.
It is important to draw a subtle distinction. The OP seems to refer to the protagonist's gestalt as "an integrated personality that is more than the sum of its individual traits". Personalitypathways.com uses the word gestalt in this fashion to describe the difference between personality traits and types:
While these faces hide the true type, they are not masks in the sense
of being false because they are a part of the overall Gestalt of type.
This is an application of the term that departs from the original principles of Gestalt Psychology, even if it is somehow consistent with it. According to its founder, Max Wertheimer, Gestalt Psychology is more about how a person perceives individual stimuli and organizes them into a pre-eminent cognitive framework, so that the whole determines the interpretation of the parts:
"There are wholes, the behaviour of which is not determined by that of their individual elements, but where the part-processes are themselves determined by the intrinsic nature of the whole. It is the hope of Gestalt theory to determine the nature of such wholes"
psychology.about.com emphasis added
Gestalt Personality Theory posits that personality is an expression of this pre-eminent cognitive framework, and that when personalities contact they should have a beneficial mutual impact on each other:
Contact: The relationship between "me" and others. Contact involves
feeling a connection with others or the world outside oneself while
maintaining separation from it.
The word impact recognizes this objective contact of subjective personalities, allowing for the fact that the speaker's pre-eminent cognitive framework determines her experience of the protagonist's personality traits, so that her impression of the protagonist is other (neither better nor worse, simply other) than the objective reality of the protagonist's personality. The protagonist is not necessarily vindaloo curry, but the speaker experiences vindaloo curry in her contact with the protagonist.