Look at our itinerary

what time do you arrive in Paris on Day 2? the way you've printed out your itinerary is a bit confusing. it seems that you're going from Frankfurt to Paris on day 16, then flying home from Paris. have you already purchased your airline ticket home? You might consider cutting out Frankfurt. If you do, then you could fly into Paris, then work your way from Paris, to Vernazza by train, stopping in Nice along the way for 2 nights, then go to Cinque Terre, then catch train to Rome for a few nights, then go by train back up to Florence for a couple nights, then Venice for a few nights and fly home from Venice. (Train travel in Italy is inexpensive) Stay in a hotel near the Venice Marco Polo airport if you have an early morning flight home. Whatever you decide have a great trip!

I see your point...Problem is we have to go to Frankfurt for 3 days as my finace has purchased tickets to see Formula 1 racing.

Also, We would probably leave Vernazza on the evening on day 10 if possible. Any other suggestions???

This looks very busy for 18 days. Venice in 1+ days is what the Rick Steves tours allow there. In that time you can hit the high points and get a feel for the city. You'll want to go back and stay longer some other time. Frankfurt looks like it belongs in another trip. If I were doing this trip I would go: Paris, fly to Rome; train to Cinque Terre (look for La Spezia Centrale on train schedule); train to Venice; fly to Paris. You can figure the flights will take up half a day; the train Rome-La Spezia takes about 4 hours; the train from La Spezia or Monterosso to Venice will take most of a day. By omitting Frankfurt you will have at least 3 nights in each city. Years ago I blitzed through Europe in a month. Most of it is a blur now. I had a railpass and traveled about every other day and hit about 8 countries. Venice was a daytrip from Florence. It depends on what sort of experience you want and how much money you can afford to spend.