What is encore cruise? $750 for 3 people is very cheap
A cruise voucher is often part of what is called an 'Encore Package'. This package is offered at the end of a sales presentation by Marriott Vacation Club to get people to come back for a second points presentation in 12-18 months in the hopes you will change your mind and buy from them the second time around.
Packages often include a stay of predetermined length (3-6 nights) at a specific (or one of several) Marriott Vacation Club properties. You will also get a block of Marriott Reward Points (often in the range of 50k to 100k) and depending on the encore package purchased - a cruise voucher for 2.
These packages are not free. You pay for them in either lump sum or interest-free monthly installments. Once paid for, you get to book your return stay. During your return stay you agree to sit through another sales presentation as a condition of getting your Marriott Reward Points (MRPs) and cruise voucher.
If you buy points at this second sales presentation, the cost of your Encore Package is applied toward your points purchase making everything described above 'free'. If you don't buy the 2nd time around, you got what you paid for - a stay, some MRPs, and a cruise voucher.
Once you have your cruise voucher, you must book through Marriott's contracted cruise agent - ICE. The value of cruise vouchers vary, but generally represent a cabin for 2 in the lowest category. You pay for upgrades to better/bigger cabins, higher decks, etc. The voucher is for base fare only. Port taxes and fees, gratuities for housekeeping (generally automatically calculated in advance, but some lines will let you adjust the amounts higher or lower based on your service experience), alcohol, specialty restaurants, shore excursions, travel to and from the port, etc. - are all on the traveler. Depending on the cruise line you are traveling on, some upgrades may be offered without cost based on your 'status' with their loyalty program.
I definitely agree with the bottomless soda package if you anticipate using more than one or two glasses a day. We have done Carnival's card and the RCL cup both. We really like RCL's model (especially with their Coke Freestyle Machines). Brilliant. Haven't tried NCL yet.
As far as whether an Encore Package (and what you get in it) is worth it, the simple answer is 'maybe'. We are currently in the middle of our first Encore Package. We have paid in full. Our return reservation for a 6 night stay has been made. We will get our 75k MRPs and cruise voucher after our presentation.
For our money, we got a nearly 1 week stay in peak season in a property we could never get into this time of year as non-owners. We probably could have rented for about the same money, but not gotten the MRPs or cruise voucher. For us, whether we buy this time around or not, it was worth it.
Your mileage may vary.