There is a built in function from the Microsoft.VisualBasic assembly. I can use it in VB like this:
Financial.Pmt((dAPR / 100) / 12, iNumberOfPayments, dLoan
As already stated you can use the Microsoft.VisualBasic assembly which provides plenty of the VB6 functionality. But to be honest if you are looking more generally at financial calculations you should consider looking at Excel Financial Functions for .NET.
Update: The library can be installed via NuGet
PM> Install-Package ExcelFinancialFunctions