I wrote a small demo of a VLC mediaplayer with 2 buttons : play and stop
and 1 slider for the volume.
The slider works when the player is playing, i.e. the volume is changing depending on the slider setting.
when it is stopped and the slider position is changed I need to click on play 2 times to make the volume changing according to the new setting of the slider. clicking 1 time only keeps the previous setting of the volume.
Here is the code :

import vlc
from tkinter import *
from tkinter import ttk
        
volume_ini=50
url='http://direct.fipradio.fr/live/fip-midfi.aac'

instance=vlc.Instance('--input-repeat=-1','--fullscreen')
player=instance.media_player_new()
media=instance.media_new(url)
player.set_media(media)
            
root=Tk()
window_height=240
window_width=320
screen_width=root.winfo_screenwidth()
screen_height=root.winfo_screenheight()
x_cordinate = int((screen_width/2) - (window_width/2))
y_cordinate = int((screen_height/2) - (window_height/2))
root.geometry("{}x{}+{}+{}".format(window_width, window_height, x_cordinate, y_cordinate))

v1=DoubleVar()


def st():
    player.stop()
 
def pl():
    global player
    global volume_ini
    player.audio_set_volume(volume_ini)
    player.play()

class BarVolume(ttk.Scale):
   def __init__(self, master):
        global v1
        global volume_ini
        ttk.Scale.__init__(self,master,
        variable=v1,from_=0,to=100,orient=HORIZONTAL,length=80)
        self.volume=volume_ini
        self.bind("<ButtonRelease-1>", self.updateValue)
        self.set(volume_ini)
       
   def updateValue(self, event):
       global volume_ini 
       global v1
       global player
       volume_ini=int(v1.get())
       self.volume=volume_ini
       labelText.set(str(volume_ini))
       player.audio_set_volume(self.volume)


 
#self.geometry("{}x{}+{}+{}".format(window_width, window_height, x_cordinate, y_cordinate))
content1 = ttk.Frame(root)
content1.grid(column=0, row=0)

led_1=ttk.Button(content1, text='play',command=lambda:pl(),width=20)
led_1.grid(column=0, row=0)
led_2=ttk.Button(content1, text='stop',command=lambda:st(),width=10)
led_2.grid(column=0, row=1)
labelText = StringVar()

lbl=ttk.Label(content1,textvariable=labelText,width=20)
lbl.grid(column=0, row=3)
echelle=BarVolume(content1)
echelle.grid(column=0, row=4)


root.mainloop()

player.stop()

I tried to play with global and local variables
I also wrote the same code with a class embedding the whole window