Can someone explain Smart Rendering and I-Frames, please?

I-frame is short for "intra-frame compression" -- meaning each frame is compressed separately. Smart Rendering is a feature of Sony Vegas to lossless export video frames from input to output if there is no filter applied, which only works correctly with I-frames.

so for Vegas users, is it wise to use the i-frame option in the same way we used "smart rendering"? i don't care about rendering times, etc.

Always use i-frame mode unless you want to compress a little more.