Good evening! I'm sieging a vaulter system in normal difficult and every time I think I'm going to destroy their defenses, they replenish their forces and another ground battle happens. What can I do to decrease fast their ground defenses?

are there any tactics I can use to win ground battle against them in one turn?

Vaulters have a population mechanic where they get free extra damage applied to ground battles similar to orbital bombardment...per population point they have on the planet. This bonus damage can be combined with the retaliation damage from defensive structures like tactical armaments making them very dangerous does to do ground battles with if you don't have Superior numbers as you will take losses and it may take time.  They can also double the damage bonus effect for 10 turns by using a luxury resource population booster. 

Also while not economically ideal, there is a system luxury upgrade that adds manpower to every battle ground battle, similar to the conscription battle tactic, but does not consume a pop.  This when paired with conscription can make a 1 turn victory almost impossible against them in a ground battle as they will recurit a significant number of infantry every turn above and beyond their normal defenses. 

There are really not many options for viable one turn victory against a heavily fortified and populated vaulters planet, but here are a few tips. 

1. Use large fleets with A2S titanium slugs that increase losses to blockaded systems until their standing army is 0.  This will mean they will only be able to field infantry using conscription and or the system development luxury resource that gives them free infantry for ground battles. 

2. Ensure you have advanced into the military tree deep enough to have collected much higher deployment limits for ground battles. Several techs will do this, but mainly you get a free boost each time you unlock a new tier.  Supiorior numbers are often the key to wiping out a force in one turn. 

3. Use the battle tactic that increases your deployment limit. You will take more losses but if you have the numbers you will always deal more damage using that tactic than the other two options. The flat damage from orbital bombardment is great when your forces are small, but it gets out classed as armies get larger. Especially if you have upgraded your forces with strategics....which brings us to...

4. Upgrade your forces using strategic resources. The bonuses are very relevant and can go a long way toward getting enough damage on the enemy to get a one turn wipe. 

5. Adapt your forces. The enemy AI will almost always go for a mix of infantry tanks and a few fighters.  This mixed force is almost always beat by a full stack of tanks due to infantry always being the bulk of AI armies. Infantry get destroyed by Tanks once they are upgraded a bit...and fighters while effective vs tanks just don't counter them as hard and the AI never brings many if they even get them. If you want a one turn wipe and victory you need upgraded tanks fully stacked and deployed in mass against a smaller total force.