We are taking a trip to Dubrovnik then some islands and then ending in Split. Then we are going to spend 4 days in Venice. I am at a total loss as to how to get from Split to Venice. Ideally we would like to fly but can only find one flight a week. Other option would be a ferry but they don't go from Split to Venice. I don't think.
I would appreciate any suggestions. Thank you.

Or we could reverse the trip and fly from Venice to Split (or close by?) and then go south to Dubrovnik and fly to Venice from there. But...still can we get from Venice to Split (or close by)

One option would be to go by bus or train to Zagreb and then on to Venice.

Did you check skyscanner.net? Croatia Airlines plus partners flies to Venice more than once a week, but the flights require at least one stop and aren't cheap. Flying Split to Zagreb is cheap, but it's a long train trip from there.

There are ferries from Croatia to Venice, but they go from ports in Istria.

The ferry leaves from Rovinj (and possibly Pula too) which is not that far from Split. Could rearrange your schedule so you are in one of those ports really early in the morning?

"not that far from Split'

A mere 500 kms...

Ferry from Split to Ancona, Italy (daily) overnight, then take the train north to Venice the next morning.

Yes like Andrew says there are three separate ferry lines operating day and night services between Split and Ancona - once you to to Ancona split by train right away for Venice.

split to ancona ferries

Split to Ancona ferries:

http://www.directferries.co.uk/split_ancona_ferry.htm

You could fly to Zagreb and use bus/transfer to Venice.

Or ferry to Pula and fly to Split.

If you want to split it up take a train from Slpit to Zagreb - six hours - stay in lovley Zagreb and go via Llubjana to Venice by train or bus. But a long long way so if in a hurry fly.

Croatia Airlines has a nonstop from Dubrovnik to Venice; I learned about it here on fodors. It leaves at 5:30 pm and arrives 7 pm - maybe not every day, but I just checked some dates in May and it seems that way.

One way is crazy expensive but booked RT and don't use the return Venice - Dubrovnik flight. Mess around with the return to get the best deal. It will cost a couple hundred euros, but it is the fastest and easiest.

don't pay with a credit card or if you don't use the mandated return ticket you may be charged with the higher one way - at least some airlines do that.

What airline does that PalenQ. Can you give examples?

You could take the seaplane from Split to Pula and then ferry to Venice.
http://www.ec-air.eu/en/seaport/timetable/

Some trans-Atlantic airlines definitely had that policy a while back - I don't recall which ones but in some cases round-trips were cheaper than one-ways and if you didn't take the return you'd be charged the one-way price and makes sense as it is an easy policy to enforce. I'm not saying most or any do it but something to consider.

-Some trans-Atlantic airlines definitely had that policy a while back-

The OP is flying from Croatia to Venice.