It's been a while since I played but I thought I'd take a look at your replays and see if I could give you any advice. First thing I noticed is that your build orders were a little sloppy. You can see in the first game on Vantage that your opponent takes a lead on metal income very quickly because they built all three mexes before building power. You should basically always build 3 mexes into 2-4 energy (1 or 2 solars worth) except for where you have unusual base setups.
Secondly your first constructor and commander are often both building different economy buildings at the same time in your base. If you are keeping both in your base, they should be building the same building to maximize economic gain early as ZK economy is very snowbally. Also you were keeping your commander in your main base for too long in both games. Your commander needs to head out on the map earlier to secure more economy or you will naturally fall behind someone else doing so.
In the second game your build order was better than your opponent's and you managed to get ahead of your opponent's income, but you made a few mistakes. You spent too much of your metal building energy, and took a long time to connect your energy grid to the mexes. You should have tried to secure the mexes near your base before building a big overdrive grid, and the stardust you built in your main was far too early. It would have been better securing more mexes first before committing to heavy defenses, especially since the layout of that map makes it very easy to defend the corner bases. Sending a con to a corner to build those mexes is defensible but you should be capturing closer ones as well at the same time with another.
In vantage defense is hard, and the key to being able to defend there is to have radar coverage so you know where the scorchers are. Getting radar in good positions on that map can be hard and will make it harder to play shields on if you don't know where the threats are since you have less mobility.
Always try to raid. You had one decent raid in the first game and could have gotten two mexes for a single bandit which would have been a great raid. Just like that it doesn't have to be a big commitment of raiders to earn value, just having threats on the opponent's side of the map interfering with their plans will lessen the pressure on you at home.
And generally, in the early game, shields rely very heavily on bandit under con shield with the occasional snitch to try to catch out groups. Right click 2 bandits onto one of your constructors to follow it around and keep it safe, and keep them under the shield as much as possible when engaging enemy scorchers. After the first few minutes increase that to 4, and it will be relatively hard for them to do much without blobbing up and ideally running into your snitches. When you start building felons, accompany them with 2 or 3 thugs each to get additional regen for their weapon, and to have something to tank for them. Keeping the thugs in front of them as you push forward will significantly reduce the amount of fire they take - and increase their offense strength simultaneously.
You will not be able to rely much on lotus past the early game since scorchers can brute force their way through with enough mass and fencers can destroy them for free, but stinger will stop fencer pushes. You will need to protect those with units/lotus/stardust/faraday though if you want to stop them from being dived by scorchers. Once fencers start coming out you should also consider mixing rogues into your armies, since they will do well vs unmicroed fencers. And always, always kite your bandits back against scorchers if you don't have a shield to hide under, scorchers really don't want to engage into you if you're moving away at full speed. I hope that helps!