Here is a link to Vassals in the wiki that has some info on "Vassal Feeding" though it is a bit out of date.
http://www.eu4wiki.com/Vassal
So the idea behind "vassal feeding" is to make conquering territory (especially territory "unlike" yours) easier / more manageable.
-New territory needs to be Cored and this costs ADM points. ADM points are usually considered more valuable / in demand than Diplomatic points.
-Conquered territory might not be of an accepted culture thus increasing revolt risk
-Conquered territory might not be of the same religion thus increasing revolt risk
Normal Flow with Morocco as an example:
01. When starting to conquer a new area (in this case Morocco and the rest of the Maghreb) you can look for a country that does not currently exist but still has existing Cores or a very small country (currently small) that you can annex in one bite. When moving into Morocco there are no small countries nearby to work with so you look for countries with cores "inside" the country you want to work on conquering first. As mentioned above Fez is an excellent candidate for this. Identify which Moroccan Provinces have Fez Cores.
02. Gain claims on as many provinces as you can that also have Fez Cores and are inside Morocco, or you can claim only one. Make sure that Fez also doesn't exist (rebels etc.)
03. Fight a war with Morocco and take all or only one province with Fez Cores. Usually you don't want take any other provinces in this first war.
04. Convert the future Capital of Fez (you have to just know which province this will be) to your Religion. This is easy if you only control one Fez core. If you do take more than one Fez Core you may want to convert more than one province to boost Religious unity in Fez when it is initially release. You probably DON'T want to convert ALL provinces because this might discourage the released Fez from taking the Religious Idea group (no provinces to convert). Here is a link to
Fez on the wiki
05. You probably DON'T want to convert the Culture of the Fez Provinces. Fez will take on the Primary culture of its capital province when released, countries get less penalties and revolt risk from cultures in the same culture group but are "non-accepted" it will get no penalties from provinces from its Primary and Accepted cultures.
06. Release Fez as a Vassal. You do this from the Diplomacy tab. This will use up a Relationship so it is a good idea to have one free unless you don't mind going over the cap.
07. Wait for your Truce to end with Morocco or start a war with another nation with a boarder with the newly created Fez. (Especially nations who own Fez cores)
08. In the piece deal you can either use "return core" if an occupied province has a Fez core or before you negotiate a peace deal you can assign occupation to Fez. (new in 1.8). As an alternative you can take all the provinces in the peace and sell Fez all of them. Fez may not buy some though (they don't "want" the province) so be careful. Also, make sure you don't Overextend you Vassal too much. (especially not over 100% over extension.) You will have to do some math in your head to calculate the amount of over extension for your vassal.
09. Your Vassal will now use
its ADM points to core the newly conquered provinces. It will also use its Missionary to Convert all the provinces it can convert to its Religion, which SHOULD be your religion (from step 4).
10. Repeat steps 7-10 until you are ready to reabsorb your now MUCH larger Vassal.
11. Open Diplomacy and assign a Diplomat to Annex Vassal. This takes time and Diplo points to do and is based off the Current Base tax of the Vassal. You
CANNOT Annex a vassal you have turned into a March. You will have to un-March any vassal and take the Stability / Relationship hit (-200) before you annex. You will also have to let a great deal of the relationship hit (-200) burn off before you get back to +190 Relations which is the minimum for Annexing a vassal. Ultimately you may not want to create Marches out of any Vassal you want to absorb.
Finally you will want to use multiple vassals for this process VS. one. Once a vassal gets to a moderate size or there is another small vassal you can create at the edge of your current vassal, create the new vassal and begin to reabsorb the current vassal.
You will also need a Casus Belli which maybe hard in earlier parts of the game if you don't share a boarder with the country you are trying to conquer once you have created a vassal to feed. This has been somewhat reduced because you can use your Vassal's CBs but they may not have one. Once you get the Imperialism CB you are pretty much set.
