There should not be a vent penetration that close to a valley.
One can install roof flashing by various steps. You can find these steps on several websites. A website you can look on for installing roof flashing is YouTube.
e-heko.co
In plumbing the roof vent should be a min of 4" to prevent hoarfrost
Your vent covers should be very easy to install it is a simple diy job. You can find an instructional video at www.youtube.com.
If there is no vent off the kitchen sink you would need to install an in-line vent.
shingle overlap when properly attached is best for valley water run-off
Why would you? If you had increased the vent terminal to 4" this will prevent hoarfrost
It is a valley
Youtube offers several videos where one may learn how to install a Soffit vent. Alternatively one may look on do it yourself websites such as DIY network and This Old House.
You have to loosen the screws on the mirrors then just slide it on
Install an automatic air vent on the line
Vent into an unused room.