Here is a script you can run on the ESXi host to add an annotation into multiple vmx files:

SAVEIFS=$IFS  
IFS=$(echo -en "\\n\\b")  
for i in \`find /vmfs/volumes/ -name \*.vmx\`  
do  
echo Filename:$i  
echo Backing up VMX...  
cp $i $i.bak  
echo Backup complete...  
echo adding the annotation:  
echo "annotation = \\"&ltFill the annotation text here without the angle braces&gt\\"" >> $i  
echo Update done...  
read -p "Hit any key to continue..."  
done  
IFS=$SAVEIFS