Using the latest version of vmime (0.9.1), encoding the following string: Jean Gwenaël Dutourd will result in:
Jean =?utf-8?Q?Gwena=C3=ABl_?= Dutourd
However, decoding this will result in Jean Gwenaël Dutourd (notice two spaces between the last 2 words). The encoder adds a _ after the second word, but since the last word is not encoded, the space between them is not ignored, and is decoded into an additional space.
This bug was fixed in revision 597.
Thanks for your report.