All true, but Ortiz isn't anywhere near Vaugh's girth the last two years of his career. Sure, Ortiz is no svelte guy, but he isn't this big.
http://msnbcmedia.msn.com/j/msnbc/Components/Photos/040108/040108_mo_vaughn_.vsmall.jpg
Mo was 34 and 35 when he played for the Mets. Ortiz is 30 this year, and if they extend him, it would be for his 31-34 age years. Provided he maintains his current size, I don't see this a big issue of concern, especially since he won't be adding stress to his knees by playing the field. Besides, even if he does experience similar problems, it will be worth a .260/.340/.450 with 26 homer line (Mo's age 34 year line) that last year to get what he's doing now for a few more years.