Sorry for the delay in response here.
We calculate VWAP the same for all symbols that have it available.
The VWAP value takes into account all trades (and all trade volumes) for the current day (resets on the first trade of the day)
A good example currently is the symbol GOOG which as of this post, only has 13 trades thus far today.
Price Size Type
1051.51 100 E
1051.3 99 O
1051.3 1 O
1051.3 100 E
1051.14 4 O
1051.23 74 O
1051.23 7 O
1051.23 16 O
1051.23 100 E
1050.89 3 O
1050.89 1 O
1041.16 4 O
1048.5 1 O
The current VWAP value is 1051.22