We know Vin removed one of Marsh's eye spikes just before she ascended.
Marsh watched limply as Vin fell to her knees. Shaking, she reached for one of his eye-spikes.
There was nothing he could do. He'd used up most of the healing in his metal-mind, and the rest would do him no good. Stored healing worked by way of speed. He could either heal himself a small amount very quickly, or wait and heal himself slowly, yet completely. Either way, he was dead as soon as Vin pulled those spikes free.
Finally, he thought with relief as she grabbed the first spike. Whatever I did . . . it worked. Somehow.
He felt Ruin's rage, felt his master realizing his mistake. In the end, Marsh had mattered. In the end, Marsh hadn't given up. He'd done Mare proud. Vin pulled the spike free. It hurt, of course—hurt far more than Marsh would have thought possible. He screamed—both in pain and in joy—as Vin reached for the other eye-spike.
And then, she hesitated. Marsh waited expectantly. She shook, then coughed, cringing. She gritted her teeth, reaching toward him. Her fingers touched the spike.
And then, Vin vanished.
The Hero of Ages - Chapter 73
(bold emphasis mine)
Despite being mentioned five times, there is no indication which spike (right or left) Vin pulled out.
The only other time this is mentioned in the book, it is also unclear which spike was removed.
Something moved behind him, and he spun, raising his blade, letting the atium lead him. Yet, he froze, uncertain. The creature behind him was no koloss. It stood in a black robe, one eye socket empty and bleeding, the other bearing a spike that had been crushed back into its skull. Elend could see straight into the empty eye socket, through the creature's head, and out the back.
The Hero of Ages - Chapter 80
If I recall correctly, Marsh had both spikes next time we see him (talking to Marasi in The Alloy of Law). So do we ever find out which spike was removed?