buttonhole cannulation with regular 15g needle?

No- regular needles can ruin the buttonhole and damage the fistula- buttonholes are only to be cannulated with special dull needles- see below/link.

Cannulating a buttonhole

Cannulating mature constant-sites in native AV fistulae using Medisystems ButtonHole® Needle Sets with anti-stick dull bevel

1. Perform a complete physical assessment of the AV fistula and document the findings.
2. Remove any scabs over the cannulation sites.
3. Disinfect the cannulation sites per facility protocol.
4. Using a Medisystems ButtonHole needle set with anti-stick dull bevel, grasp the needle wings, and remove the tip protector. Align the needle cannula, with the bevel facing up, over the cannulation site and pull the skin taut (Figure 1).
5. Carefully insert the needle into the established cannulation site (Figure 2). Advance the needle along the scar tissue tunnel track. If mild to moderate resistance is met while attempting to insert the needle, rotate the needle as you advance it using gentle pressure (Figure 3).
6. A flashback of blood indicates when the needle is in the access. Lower the angle of insertion. Continue to advance the needle into the AV fistula until it is appropriately positioned within the vessel.
7. Securely tape the ButtonHole needle set (Figure 4) and proceed with the dialysis treatment per facility protocol.

*Note: Ensure the same needle insertion angle and depth of penetration are used consistently for each cannulation of a constant-site.

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http://www.homedialysis.org/learn/buttonhole/

The above poster is correct; and just to clarify, the initial advice you were given regarding using a sharp fistula needle AWAY from the buttonhole is the appropriate course of action if you aren't equipped to access the buttonhole.

The buttonhole is a tract created by the same tech cannulating the same site on the access for about two weeks, at which point the tissues heal much like an ear piercing - once it's established, any experienced cannulator can access the buttonhole with a blunt needle (after carefully removing ALL of the scab from the site).
Imagine stabbing through a piercing with a sharp needle... if your needle doesn't go exactly through the hole already created; the subsequent tissue repair where your sharp needle pierced the tract of the piercing may cause scarring and ultimately ruin your piercing - or in this case, the pt's buttonhole.