As the system is working, (but not very well), there is no need to drain it, pull a vacuum, then refill.
The system has pressure, just not enough…
Step 1: Fill valves. Get some dish soap, and water. Use either a spray bottle or small paintbrush to put soap/water mix over the R134a valves. Look for leaks.
Very common to have pin leaks here. Valves can be replaced without draining the system, but requires a special tool…
Aupoko R134A Valve Core Quick Remover Installer, High Low Pressure Valve Core Remover Tools Kit for R13A Refrigerant Air Conditioner System
https://a.co/d/gKX7Hn9
Step 2: Get a bucket filled with lukewarm water, a can tap, and a R134a PAG oil charge…
Not a can of freon, a can of oil. Only do this ONCE in the life of the car, or you will drown the system…
Super Tech R-134a PAG Oil Charge Refrigerant, 3 oz.
https://www.walmart.com/ip/337218764
Add to A/C system…
Use lukewarm water to keep can from freezing…
I like the A/C Pros Oil Charge, 2 ounces freon, 2 ounces oil, and 2 ounces “Ice 32 Performance Enhancer”…
Get your A/C PRO R-134a PAG 100 Oil Charge with ICE32 Performance Enhancer - CERT102-1 at Blain's Farm & Fleet. Buy online, choose delivery or in-store pickup. Great prices on A/C and Refrigerants.
www.farmandfleet.com