Raw vega treats a one-point volatility move as identical in every expiration, which is wrong. Front-month implied volatility routinely swings five points while the one-year barely moves one. Adding raw vega across a calendar book therefore overstates the back-month risk and understates the front.
The usual fix scales each expiration's vega by the square root of the ratio of a reference maturity to its own. The result is a single number that behaves like front-month vega and can be compared across a portfolio.
Example: a book is long $2,000 of vega in the one-year and short $1,000 in the 30-day. Raw vega says net long $1,000. Weighted to 30 days, the one-year contributes roughly $575, so the book is net short volatility in the tenor that actually moves.
Related: vega, volatility-term-structure, position-greeks, forward-volatility