The Badei Hashulchan 189:5 (שבכל ענין) asks this question. His case is where it gets bigger by an extra day each time. He remains with the question, צ"ע.
EDIT: As for Fibonacci, the Mechaber in 189:10 has a case of 15,16,18 and says that since the last one jumped one more there is no vesst. The Shach there says that this would be the case even if there was a vesst kavua on the 14th beforehand.
This shows us that we do not set a vesst of a Fibonacci pattern, since we now have 14,15,16,18. According to the Fibonacci pattern she should be choshesh on the 21st of the next month.