M.S.
it is awful to listen to your baby cry.. but you are doing the right thing. keep the drop off time short, lingering only makes it worse for both of you. just make sure and give her a big hug and kiss, tell her you love her and that you have to go to work and will see her at the end of the day. she will go back to her normal routine soon... it is hard at that age when she gets a taste of how she might like every day to go... with the full attention of her parents every day.
as for naps, while many will disagree... if she was napping fine before without a drawn out crying before and during your vacation, I think I'd just let her cry it out. she will soon figure out that you aren't gone for good and will come back when she wakes up. I know lots of moms think this is cruel, but it is the quickest way. I am not a fan of this with younger children, but at 18 months she is old enough to have figured out what to do to make you stay in her room a little longer. another alternative is the book the No-cry sleep solution or healthy sleep habits, happy child.. they both have good tips for "correcting" changes in sleep habits.