We will be travelling from Yosimite to San Simeon (stopping at the Hearst Castle) to Vallejo in July. Are the roads (Pacific Coast Highway) O.K. to travel with a 25'trailer or will it not be worth fraying the nerves?

You'll only be on Highway 1 for part of the trip between between San Simeon and Vallejo depending on your preferred route. You'll not be anywhere near the coast between Yosemite and Hearst Castle.

Of course, the roads in California between these 3 major areas are perfectly fine for traveling with a trailer. Millions of people do it every day of every year.

I wouldn't want to drive thru SF traffic
on hwy 1
with that trailer.
consider getting on 17 at Santa Cruz and come over to hit 880 so you can take it up to Vallejo.

I woudln't take Highway 17 from Santa Cruz with a trailer. It is very winding and has some sharp curves.

To get to 880, after Monterey stay on Hwy 1 until it intersects with 156, then a short way to 101. Pick up 880 in San Jose.

I totally agree with "jtrandolph" and I am really uneasy about traveling up Highway one after Hearsts Castle..
Most is only a two lane road with some "turn-outs" and few passing lanes----however...IF you had ANY car or trailer trouble pulling off the road would be impossible and mechanical help is very limited between Castle andBig Sur-Carmel. Having said that --that section of the highway is the one of the most awesome coastal drive I have ever seen-- Happy trails..
Try getting advice from www.rv.net/forum

Stay off of hwy 17.
Use 101 to 880 to Vallejo.
Yosemite, use 140 and most direct route to S/Simeon, I think 41 is the main.
If you are inexperienced with a trailer, the roads are good, pay attention.

Don't use Hwy 1 no. of S/Simeon...go back to 101 I think around Morro Bay. Hwy 1 is scenic but very taxing and can be a bad piece of road if you are not experienced with a trailer.

Traveling south of San Simeon just a little way south of Cambria, Highway 46 will take you over to Paso Robles and highway 101.
I hate to see you miss the Big Sur area...
IF you had a place to leave your trailer..You could drive you vehicle from San Simeon up the coast to Big Sur area (about 60 miles)-take a picnic lunch or "early dinner" and drive back as the sunsets!!!
There is San Simeon State Park -no hook ups--but nice park. Of Course from May to Sept it is usually full--However you can make reservations online 5 months ahead..Check it out...
By the way where are you from?
As you might have guessed this whole area is my favorite place on the whole coast..Have gone to Morro Bay 2-3 times a year for 25 years....
It has changed a lot but I still love it...

Well 880 does not go to Vallejo. You either have to catch I-80 which will take you to Vallejo, you will go over the Carquinez Bridge or 680 which will take you over the Benicia Bridge and then west on 780 to Vallejo..and depending on where in Vallejo you are going to might have to catch I-80 at the end of 780.

880 connects to 80, yes.

Sorry. My assumption was the OP was going to try to take 1 all the way up into SF and then maybe 101 to 37.
The other suggestions are better than 17, but they won't see much coast.