To get the vacation scripts working, I had to modify lib/sieve-php.lib:

-fputs($this->fp, "AUTHENTICATE \"PLAIN\" \{$this->len+}\r\n");
+fputs($this->fp, "AUTHENTICATE \"PLAIN\" {" . $this->len . "+}\r\n");

and

-fputs($this->fp, "PUTSCRIPT \"$scriptname\" \{$len+}\r\n");
+fputs($this->fp, "PUTSCRIPT \"$criptname\" {" . $len . "+}\r\n");

At least, the { is escaped whereas the } is not, but it looked like removing the backslash dit not completely solve the problem, and separating the len and the + did help finally.