Assuming you're using H.264 for exporting, the bitrate is the main setting concerning the quality of your video. Increase it to raise the video quality and reduce compression artifacts at the cost of a larger file. Using VBR 2-Pass will yield better quality at the cost of longer rendering times compared to exporting with the same bitrate with VBR 1-Pass.
There is no standard per se (at least for internet video, there is for other media such as DVDs), but here are a few things to consider. First, raising the video bitrate higher than the original bitrate of the video that came out of your camera won't yield you any quality improvement. Personally, I haven't been able to notice any perceivable difference above about 6 Mbit/s for Full HD video at 25fps. Of course, that value has to be adjusted depending on your resolution and framerate (see Does raising the framerate while not changing the bitrate result in quality loss?). So if your video is e.g. shot at 50fps, I would recommend about 12 Mbit/s. However, this is only my personal experience. Another example is Youtube, their Full HD video is compressed using a bitrate of less than 1 Mbit/s (so that even people with slow connections can watch their videos without stuttering), but that does look pretty poor on a large monitor. I would suggest you experiment with by exporting a small portion of your video with consecutively higher bitrates until you don't see any compression artifacts and use the last value for your full export.
Now for the settings in Premiere Pro, maxing out the Maximum Bitrate won't do much as the primary controller for the video bitrate is the Target Bitrate setting. With VBR (1- or 2-Pass), the encoder will increase and decrease the bitrate over the running time of the video as needed while trying to achieve an avarage bitrate that equates the Target Bitrate setting. The Maxium Bitrate is the ceiling for that margin, i.e. the Maximum Bitrate you set will never be exceeded.
If you don't see any problems with your video after uploading it to Vimeo, you can safely ignore the warning. If it still bothers you, just export it again using a Target Bitrate above their suggested setting.
