Basically copy coupon, view price history or create e-mail price alert. What is the best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest coupon code price check (UTC time):
Vacuum, Power (W): 400W, Cleaning Area (sq.m.): 380㎡, Ceramic Tile, Voltage(V): DC 4.5-18V, Tile Floor, Suction, Wood Floor, Noise (dB): ≤78dB, Function: Dry, Feature: Auto, Dust Box Capacity: 0.5L, Type: Upright, Marble Floor, Floor Types: Carpet, Frequency (Hz): 50 – 60Hz, Shag Carpet
Price history and price alert
Here is price history (for all/most countries) for Xiaomi DREAME V9P Vacuum Cleaner.
Price records source(s): Gearbest
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).