I have got about 5-7 vassals without fiefs, but with hight relation rate (about 60+ each). Should I give them fiefs or they won't leave me anyway? I have got 3-4 vassals with low relation rate (about 4-8) and if I start to giving aways fiefs I could lose relationship with them.
Politics in Calderia is very very tricky. There are benefits and drawbacks of doing either, and you should be very careful when granting fiefs as you can easily alienate a lot of your vassals if you give them out too fast.
Here are the things you have to consider:
I prefer to stick to a small but highly dependent group of lords, but this leads to other issues as you expand in territory it becomes harder to patrol and defend your borders. The more lords you have, the more annoying it becomes, so the more time you'll have to spend patching up relationships.
First of all, use diplomacy mod instead of native. This allows you to set a policy which can affect relations with your vassals (both good and bad), this will make time your friend.
As to assigning them fiefs, each lord should have at LEAST have 1 fief; if they have no fiefs then their relations will keep dropping until they defect, even if the lord loves you when he joins.
As stated above, never give a lord 2 fiefs which are far away from each other, because the lord will neglect one of them.
And, as stated above, keep your castles and towns for good lords, these are the lords that have a high relationship with you without your active attempt to raise it. This depends on your own character and the way you rule your nation.
Example: if you have a very high honor, honorable lords will start liking you, even if they never met you. They still lose relations with you if you give them fiefs, but given enough time this problem fixes itself. Give your important fiefs to these lords as they are very unlikely to rebel.
Another "fix" for the problem, if you have lords with -1 or lower you can ask a friendly lady to send them a gift. It's very expensive, but it does do the trick.
-20 to 0 = unhappy, these lords are unlikely to respond to your orders, such as gathering the army or coming to a certain spot.
-100 to -20 = disgruntled, if this lad has an important fief you might want to make the first move, when they go below -20 there is a very real chance of them rebelling, the lower the relations the higher this chance. If they hold a castle or town, you lose this with all of the villages that come with it.