OK, think this will work:
find /home/vmail/ -type d -path "*/*/Maildir/.LearnAsSpam/cur" -exec sh -c '(cd {} && mv ./* ../../.Junk/cur)' ';'
Just in case anyone is interested, the whole script for spam learning and moving spam looks like this:
# do a spam check in LearnAsSpam and LearnAsHam folders
# find spam
sa-learn -p ~/.spamassassin/user_prefs --spam /home/vmail/*/*/Maildir/.LearnAsSpam/{cur,new}
# find ham
sa-learn -p ~/.spamassassin/user_prefs --ham /home/vmail/*/*/Maildir/.LearnAsHam/{cur,new}
# move spam
find /home/vmail/ -type d -path "*/*/Maildir/.LearnAsSpam/cur" -exec sh -c '(cd {} && mv ./* ../../.Junk/cur)' ';'
find /home/vmail/ -type d -path "*/*/Maildir/.LearnAsSpam/new" -exec sh -c '(cd {} && mv ./* ../../.Junk/new)' ';'
# move ham
find /home/vmail/ -type d -path "*/*/Maildir/.LearnAsHam/cur" -exec sh -c '(cd {} && mv ./* ../../.Inbox/cur)' ';'
find /home/vmail/ -type d -path "*/*/Maildir/.LearnAsHam/new" -exec sh -c '(cd {} && mv ./* ../../.Inbox/new)' ';'
this was added to the /etc/cron.daily/spamassassin script