Proof of yellow-fever vaccination is only required if you are coming to Cabo Verde from an infected area.
All visitors to Cabo Verde (except holders of some African passports) require a visa. A one-month tourist visa can be obtained without any problems on arrival for €25.
Technically, there's a fine of CVE15,000 if you let your Cabo Verdean visa expire; in reality, if you're only a little over nobody is likely to care.
For an extension you need, in theory, to fill in a form, supply a photo and lodge the application at the Direcção de Emigração e Fronteiras; in reality, staff members here are likely to be highly confused if you turn up requesting an extension!
The one-month tourist visa for Cabo Verde is available on arrival at the airports and at the ports of Praia and Sal. The €25 fee is payable in euros only – it helps to bring the exact money.