In general, the file is encoded to meet the target bitrate. Based on the file sizes it sounds like your Vimeo setting is about twice the automatic setting. Note that the recommended Vimeo bitrate is for submitting the file prior to recompression. The final Vimeo bitrate is about 2500 kbps for a 1280x720 file.
I am having trouble with the math using the numbers in your post. The bitrate for the automatic setting can be calculated as follows:
Bitrate = File Size / Duration
= (180MB) / (215s) x (8 b/B) x (1024 kb/Mb)
= 6858 kbps
This seems high for a 1280x720 automatic setting. What is the frame rate?
The encoded bitrate using the Vimeo setting is as follows:
(351MB) / (215s) x (8 b/B) x (1024 kb/Mb) = 13,374 kbps
This is much higher than 7000 kbps.
Please confirm the file sizes and length of video. If the numbers in the original post are correct, please take screen shots of the export panel with the automatic and Vimeo settings.