The first time that voice actress Nicole Jaffe uttered the above-quoted statement, she wasn't acting as Velma Dinkley. Instead, Nicole was talking about her own eyesight.
From Behind the Voice Actors:
"Like her character, Jaffe was myopic and needed glasses or contacts to see. At the first voice recording rehearsal for Scooby-Doo, Where are You!, Jaffe accidentally dropped her glasses and cried out something to the effect of 'my glasses! I can't see without them,' which became a trademark gag and catch phrase for Velma."1
Nicole left acting in 1973, with Velma being her only voice role.
Since then, Nicole has become a successful Hollywood talent agent, having represented people such as Patrick Swayze, Whitney Houston, Alicia Keys, Elijah Wood, Emma Thompson, Jaden Smith, Julia Andrews, Melanie Griffith and Rihanna.2.
Information Sources:
1Nicole Jaffe. Behind the Voice Actors. Retrieved from http://www.behindthevoiceactors.com/Nicole-Jaffe/
2Fleming, M. (2013, February 4). Nicole David Exits WME; Top Talent Agent Who Voiced ‘Scooby-Doo’s Velma Eyes Next Chapter. Deadline. Retrieved from http://deadline.com/2013/02/nicole-david-exits-wme-top-talent-agent-who-voiced-scooby-doos-velma-eyes-next-chapter-421574/