Should I attempt to offset larger unrealized gains in non-retirement stock index funds so far this year by smaller realized losses from earlier in the year. If so, how?

That's really not a tax question and depends a great deal on your particular situation and also on whether you think it makes sense to sell some part of your holdings now just in order to realize the gains so they can be offset by those realized losses. 

The IRS allows you to match up your gains and losses for any given year to determine your "net" capital gain or loss. If you end up with a net loss, you can use up to $3,000 per year to reduce your taxable income. Any additional losses can be carried-forward into future years, to offset either capital gains or another $3,000 in ordinary income. Whether or not this is worthwhile for you I couldn't say. 

You also have to take account of the fact that you'll be selling assets that you may wish to hold longer term.

@eric-kathe 

You should also check "Estimated" Vanguard year-end distributions vs what various Vanguard funds you actually own:

https://advisors.vanguard.com/insights/article/estimatedyearenddistributions

____________________

For the Vanguard funds that you do own, perhaps the LT distributions from those funds will offset your prior losses without you having to do anything.  But it really depends on what funds you own since it looks like a number of them do not plan to issue any year-end LT gain distributions, while others will be distributing considerable per-share amounts.