That's a crazy itenary. I assume you must fly to FCO instead of VCE, but you just bought a day on the train, instead of a day in Italy. You will arrive FCO, train to the train sta. then train for 5 or 6 or 8 so hours to VCE.

When you get to VCE, they have free luggage carts. Do not hire a porter in venice. they are very expensive. €5-6 per bag. After detraining, turn left across the front of the trains, then left again to the handicapped exit(not well marked) to avoid going down all those steps in the front of the train sta. Use the cart to take your luggage to the TAXI stand(to the left as you leave the sta), or to the correct vaporetto stand(there is more than one). If you take a water taxi, get a price first. I think a vaporetto ticket is more like €11 a day, and €60 a week. If staying a week, the one week vaporetto pass is actually a Visit Venice Red card. If you plan to go to the Ducal Palace, the ... church, and a half a dozen other places, look into one of the Venice cards, as the charge for many sites is about €10 each.

You do not loose your tickets if you miss your train. The seat res is specific, and a separate document. Make certain you stamp your tickets when you get on the train.

I prefer first on the IC's as opposed to the ES, as they are much less expensive and not that much slower. We also carry food and wine aboard.