Vancouver Canada to Las Vegas and California
 

I am planning a roadtrip in Mid September and I am looking for some constructive feedback on our plans. It's our first so we need some good feedback!

Travellers - 2 (dating couple)
Length of trip - 12 days (with 2 days extra if we run into any trouble)

Day 1 - Vancouver to Twin Falls or Salt Lake City (I want to drive to SLC, get the most driving done Day 1 but I have to check with my boyfriend as he is the main driver for the trip)
Day 2 - SLC to Grand Canyon (We are thinking South Rim but we are open to North Rim if it would be easier)
Day 3 - Grand Canyon to Vegas (Half day in Grand Canyon, heading to Vegas later in the day)(Could possibly spend 2 nights here so we get a full day if it is necessary. What do you think?)
Day 4/5 - Vegas (we might just do one full day in Vegas and leave on the 5th. We were just there in January so we don't need too much time there)
Day 6 - Vegas to California
Day 7/8 - Los Angeles
Day 9 - Los Angeles to San Francisco
Day 10 - San Francisco to Crescent City (Will spend the morning and afternoon in San Fran. If we do just one full day in Vegas, we might spend 2 nights in San Francisco so we can see more there)
Day 11 - Crescent City to Oregon
Day 12 - Oregon to Vancouver

We plan on doing the majority of our trip in Los Angeles as we are visiting friends.

His car, your car or a rental car? Try for Ogden the first night instead of SLC.
While going through southern Utah, try to see Bryce Canyon, Zion or both before deciding whether to go to the north or south Rim of the Grand Canyon.
Since you've already seen Las Vegas, stop and see Hoover Dam and stay at the Hoover Dam Lodge about 3 miles from the dam.
Where in California for day 6?
The night of day 8 you should spend in Santa Barbara or somewhere north of there along US 101.
If you are going up I-5, your Oregon stop should be in or near Eugene.

Since you seem to be interested in Las Vegas, possibly southern Utah national parks and the Grand Canyon, why not just fly to Las Vegas and rent a car to tour that area and then fly back from Las Vegas?

W/ that itinerary you will (literally) spend more time in the car than actually <i>being</i> anywhere.

For example SLC > the Grand Canyon > Vegas will take 13-14 hours wheels turning, so really two full days in the car w/ rest stops/meals, and you want to squeeze in a visit to the Grand Canyon.

If you only get as far as Twin Falls on day one then you are talking 16-17 hours in the car over the 2nd and 3rd days.

I'd fly into Phoenix or Vegas and just do the SW parks/Vegas/LA/SF and fly home - and even that will be pretty rushed.

SF is just an overnight. Drop it.

LA is just an overnight. Drop it.