VGA with NIOS II

Nios II can write to a frame buffer but it will not be able to keep up with the pixel speed to the screen. Typically you implement multiple frame buffers so that one can continuously be displayed while the CPU is populating the next frame.

Normally you use a DMA or a video controller that has an integrated DMA (master) that can fetch the pixels from the frame buffer. So you would have Nios II tell the DMA/controller where the frame buffer lives in memory and let it fetch the completed frame. If there isn't another frame ready by the time the DMA/controller is done then you just tell it to display the same frame again. I'm not familiar with the IP you are using but maybe you can re-use this design I made for the NEEK LCD output using the mSGDMA IP I designed a while back: http://www.alterawiki.com/wiki/modular_sgdma_video_frame_buffer

You'll need a different controller than I'm using since mine is meant for a 800x480 screen, but all you need is something that can accept a stream of data.