When vacuum is used during labour, a distinct mark may form on the head along the margins and is called a vacuum caput. However such marks do not last long and even a caput or swelling usually resolves by 7 to 10 days of age.
If the extraction of the baby is difficult, or trauma occurred, it is likely to result in a cephalhematoma
which is a blood swelling. This can take upto 8 weeks to resolve completely.
In such cases, an underlying fracture
should always be ruled out.
In your baby's case, the mark is square shaped rather than circular. Also it has been a month now. So unlikely to be a caput. One possibility is a cephalhematoma and other a hemagioma.
Kindly get your baby evaluated with a Neonatologist for further work up.