We've been the #1 net positive migration market for some time with our great weather, no taxes, and less-restrictive environment.

Per this NY Times article, the average American knows around 600 people.https://www.nytimes.com/2013/02/19/science/the-average-american-knows-how-many-people.html#:~:text=The%20average%20American%20knows%20about,do%20you%20know%20named%20Kevin%3FPer the US Census Bureau, the average American moves 11.7 time in their life, which based upon an approximate lifespan of 84 years, works out to be about every 7 years.https://www.census.gov/topics/population/migration/guidance/calculating-migration-expectancy.html#:~:text=Using%202007%20ACS%20data%2C%20it,one%20move%20per%20single%20yearSo, if the average American knows 600 people and they each move about every 7 years, that means that the average American knows around 85 people that move in any given year.How many of those moves do you want to be involved in?

Specifically migration of labor.

Contrary to the headlines California is actually still gaining in population due to births and international migration, so it is up to you if you want to sell, but summer in Palm Desert can be rough because you are asking buyers to come see your house when the temperature outside is 100+ degrees.

Should I be focusing on vacancy rates, net migration, job growth..?

Jonathan,In general, you should expect SFRs to appreciate more than small MFs (duplex, fourplex), that said, why folks migrate to small MFs from SFRs is cash flow (yield) is typically better per $ spent.  

We fish the Bitterroot and other rivers a few times a year and Hamilton weather is not to bad.my main point is a lot of these articles are based on % growth not actual numbers and when you start with states with such low populations does not take much in migration to move the % needle..   

In reality, no one knows (and I certainly don't), but the data doesn't point to any looming crashes.Here's a great snapshot comparing the market today to the last bubble:Comparing the Last Denver Bubble to Today2002-20062012-2016Homes for Sale at EOY (End of Year)5 year average of 22,2585 year average of 5,726Average Months of Inventory6.1 months1.8 monthsNew Homes BuiltTotal of all 5 years 93,000Total of all 5 years 30,000Cumulate Job Growth85,892180,708Net Migration to Denver24,000190,000Homes Built : Migration3.9 homes / 1 person0.2 homes / 1 personResultBubble + correction due to too much inventory and loose lendingScarce inventory drives price increasesData Source: Lonie Glessner at NOVA Home Loans

What are the local economic and population migration numbers....etc?

You also want to invest in states where there is a big migration and influx of jobs and companies, lot of job growth and population growth, pro-business and low cost of living.