Vikings didn't get into stamping their own coins until the 10th century, and even then it wasn't widespread for a while. Prior to that, they traded pieces of silver, which might be hacked up jewelry (hacksilver), ingots, globs, or whatever, and everyone who would accept silver had a set of scales to measure the weight of it.
They recorded deals made but not yet fully transacted on Tally Sticks. That is, two men would agree on a deal, then they'd carve runes into a stick that explained the terms of the deal. The stick would be split along the runes and each man would take one half until the deal was concluded, at which time the one who owed the other would get the second half. In modern parlance, the Tally Stick is a loan agreement.
Otherwise, they bartered.
They did use gold, but it was much less common at the time. Like silver, they weighed it instead of using coins.
tl;dr: If you want to still use coins and have silver be ubiquitous as the coinage, replace the word 'platinum' with 'gold' and 'gold' with 'silver', and make coppers a very small weight of silver, calling it a penny or penninger (the actual term they used) instead of a silver piece.