There is a limit on the height of the index trees in a Vision file.  The height limit is 30, which implies about 1 billion records assuming 2 keys per index node.  In practice you will likely have more keys per node than that so the number of records can be very large.  Vision does maintain a reference number for unique records in the case of duplicate key values.  This number is incremented with each write operation and has a maximum of 2^31-1 or 2.14 billion.  Both limits are well above your record count of 2.5 million records so you should be fine.

Vision 5 files support up to 64K 2GB data segments and the same number of index segments, so file size limits should not be an issue for you either.