You set the thumbnail write parameters with the -o option. This is a pattern which the input filename is pasted into to produce the output filename. For example:
$ vipsthumbnail fred.jpg jim.tif -o tn_%s.jpg
For each of the files to be thumbnailed, vipsthumbnail will drop the extension (.jpg and .tif in this case) and then substitute the name into the -o option, replacing the So this example will write thumbnails to stn_fred.jpg and tn_jim.jpg.
If the pattern given to -o is an absolute path, any path components are dropped from the input filenames. This lets you write all of your thumbnails to a specific directory, if you want. For example:
$ vipsthumbnail fred.jpg ../jim.tif -o /mythumbs/tn_%s.jpg
Now both thumbnails will be written to /mythumbs, even though the source images are in different directories.
Conversely, if -o is set to a relative path, any path component from the input file is prepended. For example:
$ vipsthumbnail fred.jpg ../jim.tif -o mythumbs/tn_%s.jpg
Now both input files will have thumbnails written to a subdirectory of their current directory.