Not sure whats going on here exactly but it seems to me that when you render a video at 25 fps, it may appear as lower when actually playing it. After all, its kinda like when you have 500 GB of External Drive storage and only 460 something GB is usable since the other 40GB or so runs the system behind it, like if Windows is preinstalled in a Drive marketed at 500 GB.
Plus why 25 fps if I may ask? Not sure if you are running an SD or HD quality video but if its HD, usually I would run it at 59-60fps whenever I render any of my HD videos for my YouTube channel. I wish you best of luck, try rendering at 30fps to see if VLC runs it at over 25 fps. There are plenty of rendering format options you can choose from in Vegas PRO.