There is no active cable or adapter that directly converts a VGA (analogue) signal to DisplayPort (digital) to connect the VGA output of a KVM switch to the DisplayPort input of a monitor. Common adapters go from DisplayPort to VGA, not the other way round.
To use a display with DisplayPort input from a VGA source, you would need an active VGA to DisplayPort converter, which is a more complex and expensive electronic device, often sold separately. This type of converter adapts the analogue VGA signal into a digital DisplayPort signal.
As far as image quality is concerned, the VGA signal is limited in resolution and quality by its analogue nature. Your old 24" VGA screen displayed WUXGA 1920x1200. With a VGA to DisplayPort converter, the 27" QHD 2560x1440 screen will be able to display the maximum resolution, but the quality will be limited by the VGA source, so will probably be less sharp than with a native DisplayPort or HDMI source.
In short, to keep your VGA switch, you need an active VGA to DisplayPort converter. It's not just a passive cable or adapter. The image will depend on the quality of the converter and the original VGA resolution, which means it will be less than the screen's full QHD capacity.
You can connect the laptop (MacBook) directly to the monitor via the USB-C port to transmit video. However, the monitor's USB-C port only provides up to 15 watts of power, which may not be enough to charge the laptop, depending on the specific requirements of your device.
The monitor requires a power supply of 100 to 240 VAC at a frequency of 50 or 60 Hz, with a maximum current consumption of 1.6 A. In operation, it consumes around 23.9 watts, while the maximum consumption is 85 watts.
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The package also contains a display port cable. You can connect the monitors together using one of the display port (be sure to use the correct port on the monitor, check for Daisy Chain) and then if you have the correct adapter you can use the second display port cable to connect master monitor to your laptop/workstation. In case you have no display port support/adapter you need to buy an extra hdmi cable