Summoners are one class that I'm not 100% familiar with, so I'm looking for optimization options to improve his capabilities:
I came up with this potential idea: Adding VMC cleric to Unchained Summoner (with suitable extreme alignment)
Is this combo worth it (aside from gaining the VMC cleric abilities) to get Sacred Summoning feat to change full round summoning spells into standard action summoning spells?
Would gaining the Summon Good/Neutral/Evil monster feat help expand the quantity/quality of the list of monsters able to be summoned by Sacred Summoning?
What other options should a summoner have to optimize things?
What is a VMC Cleric?
Variant Multiclassing from Pathfinder Unchained.
... Don't Summoners already get a standard action Summon Monster Spell Like ability? Why would you double down with standard action summon *spells*, too?
Can't have spell like ability running at the same time the eidolon.
Technically, you can. You can't use your SLA while the Eidolon is present, but you can summon the Eidolon while you have an SLA monster active (takes a minute, but with min/level, there's still plenty of time left on the SM).
Disclaimer: Don't use this trick in combat, or your GM will start throwing things at you. A CRB to the head hurts! At most, use it so summon monsters like the Bralani Azata to heal your Eidolon for free.
That out of the way, you're using up all your feats that way. Spell Focus, Augment Summoning, Sacred Summoning, and probably either Superior Summoning or Evolved Summoned Monster.
Another thing to consider is that you're already in danger of overshadowing the martials with your eidolon. For normal summoner's that's ok because getting hasted in turn 1 of every combat makes one feel awesome even if the mere class feature next door does more damage. But if you focus on summoning, there's a very high chance that you'll drown out all the martials.