Can you force a vassal to re-spawn

You may have decided to support the claimant a bit early. There's really not much you can do when all you have are 90 units with 1 lord vs 600 and 9 lords unless your party's stats (like surgery/first aid) are super high already.

Here are some tips that may help you out:

1. A max army size of 90 means that you don't have enough Renown and Leadership. A good army size to have when declaring for a claimant is at minimum of 150. Next time you should try to boost your renown (by taking fewer units into battle against a higher number of enemies). Having a renown of 1500 to 2000 is probably a minimum before striking out on your own. Also, put more points into leadership/charisma.

2. Try to build up relationships with the lords of the faction your claimant is from (capturing and releasing them repeatedly will gain you relationships fairly quickly as well as improving your honor). If you do that, you can visit their lords while they are in their castles and persuade them to join your cause. This will increase your holdings and lords without having to fight them. Since you'll always be the marshall of your faction, you can then ask your lords to join you in battle.

3. Use only high tier units and play to their strengths. For the Nords, their best units is huscarls, but they aren't mounted so suffer when fighting in the open. Your best bet is to fight in moutnaineous terrain and tell all your units to hold out on top of a hill. This will drastically slow any mounted units heading at you, giving your huscarls plenty of time to throw their axes for tons of damage.

4. Try to wait until there's a war between Swadia and someone else, when their leaders are distracted, go in and take their territory. Also, wait until you see Swadia conquer a city, this is the best time to go and take that city from them as it won't be heavily defended.

5. There are also some really cheap tricks you can do to take a castle without any losses, but that requires you to get some rhodok sharpshooters first. Just tell them to use their melee weapons and defend at the start of a castle battle. They'll be nearly invincible when hiding behind their board shields. Then wait until the enemy runs out of missiles, then tell them to open fire. After your guys run out of missiles, retreat, and repeat.

You can talk to your minister to send an envoy, who's gonna declare a political war on your enemy.

From the inofficial wiki

"By talking to this minister you can conduct many different affairs of state. Diplomats can be dispatched to the other factions, performing such actions as declaring war..."

Sadly, it would still cause problem with your honor rating and lord relations. It only prevents you from losing relationship with city/village/lord you wolud have to harras otherwise.

In order to declare war without consequence, you have to wait for event informing about border incident.