A few things to note about the Rome-Venice night train:

1) It makes about seven to ten stops between the two cities, depending on the specific day, which may or may not disrupt your sleep.

2) The vagone letto deluxe doesn't have its own private bathroom, though it does have a sink. There is a shared bathroom in the corridor.

3) It arrives in Venice at about 5:30 or 6:30, again depending on the day, which isn't actually a bad time to be in Venice but will be too early to check into your hotel to rest and/or freshen up, unless you book the room for the preceding night (and let the hotel know that you won't check in until the following morning so that they don't cancel your booking as a no-show). That would negate any cost savings of doing the night train as a hotel substitute. But you may be one of those people who can get off a train at that hour and be raring to see the town.

If any of that is a deal-breaker, you could leave Rome about three hours earlier, instead of at 10:30ish for the night train, and arrive at Venice Santa Lucia before midnight. There is vaporetto service after midnight, though it's limited, or you could take a water taxi from the station. A water taxi isn't cheap, but it would still be cheaper than the night train plus an extra hotel night in Venice. If you do that, still make sure that your Venice accommodations have 24-hour check-in and alert them to your late arrival. Or, if you're an extreme morning person, you could leave Rome very early in the morning and still get to Venice with a decent amount of time left in the day.