What is the use of GDI+? Can I use it for creating custom controls to use in Vector Graphics program?
GDI+ is used to draw. If you need some special paintings for the 'image' to show in your control that is good.
If you need to create some controls with 'unusual' forms, maybe you need to go for WPF.
To know better about GDI+ start reading:
http://www.codeproject.com/KB/GDI-plus/GDI_.aspx (info about the API)
http://www.c-sharpcorner.com/uploadfile/mahesh/gdi_plus12092005070041am/gdi_plus.aspx (how to use from C#)
Keep reading the FAQ from http://www.bobpowell.net/faqmain.htm
Then goto http://msdn.microsoft.com/en-us/library/aa289159(VS.71).aspx
Hope this helps