calling a function on dataframe data

I am not sure what I am doing wrong here, I am simply trying to call a function with a if-then-else filter in it and apply to a dataframe.

In [7]:
df.dtypes

Out[7]:
Filler     float64
Spot       float64
Strike     float64
cp          object
mid        float64
vol        float64
usedvol    float64
dtype: object

In [8]:
df.head()

Out[8]:
          Filler  Spot  Strike cp  mid   vol  
    0       0.0   100      50  c   0.0   25.0   
    1       0.0   100      50  p   0.0   25.0   
    2       1.0   100      55  c   1.0   24.5  
    3       1.0   100      55  p   1.0   24.5   
    4       2.5   100      60  c   2.5   24.0 

I have the below function:

def badvert(df):
    if df['vol']>24.5:
        df['vol2'] = df['vol']*2
    else:
        df['vol2'] = df['vol']/2
    return(df['vol2'])

Which I call here:

df['vol2']=badvert(df)

Which generates this error message:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-14-bbf7a11a17c9> in <module>()
----> 1 df['vol2']=badvert(df)

<ipython-input-13-6132a4be33ca> in badvert(df)
      1 def badvert(df):
----> 2     if df['vol']>24.5:
      3         df['vol2'] = df['vol']*2
      4     else:
      5         df['vol2'] = df['vol']/2

C:\Users\camcompco\AppData\Roaming\Python\Python34\site-packages\pandas\core\generic.py in __nonzero__(self)
    712         raise ValueError("The truth value of a {0} is ambiguous. "
    713                          "Use a.empty, a.bool(), a.item(), a.any() or a.all()."
--> 714                          .format(self.__class__.__name__))
    715 
    716     __bool__ = __nonzero__

ValueError: The truth value of a Series is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

My gut tells me that this is a simple "syntax" issue but I am at a loss. Any help would be greatly appreciated