Why didn't the Twilight vampiros wear sunblock to block the sun on sunny days?

Any form of lotion, because of the extra oils that don't absorbe into the skin, would increase their skin cells' index of refraction, causing them to sparkle even brilliantly.

Because sunblock/screen is a misnomer. It does not block UV rays from hitting the skin, it just limits the amount that skin absorbs. And even then, sunblock/screen doesn't always work -- that's why it's possible to get sunburn while wearing sunblock/screen.

So, a vampire wearing sunscreen wouldn't stop them from sparkling in the sun, as the UV rays would still hit their skin.

Like others have said, sunscreen doesnt always help against UV rays (but it DID work in The Chronicles of Vladimir Tod, then again those vamps burned in the sunlight). It probably WOULD help, but not alot. They would probably still sparkle, just not as brightly as normal

This would not work (as proved por other answers), but cloudy days to not completely block sunlight.

Because they sparkled...they didn't burn in the sun....duh.